Accounting Policies [Abstract] Accounting Policies [Abstract] Accounts Payable, Current Accounts payable Accounts Payable, Related Parties, Current Amount payable to Carrier and its affiliates, net of receivables Accounts Receivable, Net, Current Accounts receivable, net Accrued Expenses and Other Current Liabilities [Member] Accrued Expenses and Other Current Liabilities [Member] Accrued expenses and other current liabilities Accrued Liabilities, Current Accrued expenses and other current liabilities Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment Accumulated depreciation and amortization Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member] Cash Flow Hedging Instruments Accumulated Net Investment Gain (Loss) Attributable to Parent [Member] Available-for-sale Securities Accumulated Other Comprehensive Income (Loss) [Line Items] Accumulated Other Comprehensive Income (Loss) [Line Items] Accumulated Other Comprehensive Income (Loss), Net of Tax Ending balance Beginning balance Accumulated other comprehensive loss, net of tax Accumulated Other Comprehensive Income (Loss) [Table] Accumulated Other Comprehensive Income (Loss) [Table] AOCI Attributable to Parent [Member] Accumulated Other Comprehensive Loss Accumulated Foreign Currency Adjustment Attributable to Parent [Member] Foreign Currency Translation Adjustment Additional Ownership Interest Percentage Acquired Additional Ownership Interest Percentage Acquired Additional ownership interest acquired Additional Paid in Capital, Common Stock Paid-in capital Additional Paid-in Capital [Member] Paid-In Capital Additional shares issued to employee stock purchase plan for dividend reinvestments. Additional Shares Issued To Employee Stock Purchase Plan For Dividend Reinvestments Additional shares issued resulting from cash dividends reinvested in common stock Adjustments to Additional Paid in Capital, Share-based Compensation, Requisite Service Period Recognition Share-based compensation Adjustments to Additional Paid in Capital, Income Tax Benefit from Share-based Compensation Excess tax benefit from share-based compensation Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract] Adjustments to reconcile net income to net cash provided by operating activities: Advertising Costs, Policy [Policy Text Block] Advertising Costs Advertising Expense Advertising expense Allocated Share-based Compensation Expense Share-based compensation expense Allowance for Doubtful Accounts Receivable, Current Allowance for doubtful accounts Amendment Flag Amendment Flag Amortization of Intangible Assets Amortization expense related to finite lived intangible assets Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount Anti-dilutive stock options excluded from earnings per share Assets Total assets Assets [Abstract] ASSETS Assets and Liabilities Eliminated upon Consolidation [Abstract] Assets and Liabilities Eliminated upon Consolidation [Abstract] Assets and Liabilities Eliminated upon Consolidation [Abstract] Assets, Current Total current assets Assets, Current [Abstract] Current assets: Available-for-sale Securities Available-for-sale securities Award Type [Axis] Award Type [Axis] Balance Sheet Location [Axis] Balance Sheet Location [Axis] Balance Sheet Location [Domain] Balance Sheet Location [Domain] Board Approved Quarterly Cash Dividend Date Effective Month And Year Board Approved Quarterly Cash Dividend Date Effective Month And Year Date new dividend rate will be effective Board approved quarterly cash dividend rate applicable to April twenty eighteen. Board Approved Quarterly Cash Dividend Rate Applicable to April Twenty Eighteen Cash dividend Building and Building Improvements [Member] Buildings and Improvements Business Acquisition [Line Items] Business Acquisition [Line Items] Business Combinations [Abstract] Business Combinations [Abstract] CANADA CANADA Capitalized vendor rebates Capitalized Vendor Rebates Capitalized vendor rebates Carrier and Its Affiliates Carrier and Its Affiliates [Member] Carrier and Its Affiliates Carrier Enterprise One [Member] Carrier Enterprise One [Member] Carrier Enterprise I Carrier Enterprise Two [Member] Carrier Enterprise Two [Member] Carrier Enterprise II Cash and Cash Equivalents, at Carrying Value Cash and cash equivalents at end of year Cash and cash equivalents at beginning of year Cash and cash equivalents Cash and Cash Equivalents, Period Increase (Decrease) Net increase in cash and cash equivalents Cash and Cash Equivalents, Policy [Policy Text Block] Cash Equivalents Cash Flow Hedging [Member] Cash Flow Hedge Cash Flow Supplemental Disclosures [Line Items] Cash Flow Supplemental Disclosures [Line Items] Cash Flow Supplemental Disclosures [Line Items] Cash Flow Supplemental Disclosures [Table] Cash Flow Supplemental Disclosures [Table] Cash Flow Supplemental Disclosures [Table] Cash Flow, Supplemental Disclosures [Text Block] SUPPLEMENTAL CASH FLOW INFORMATION Chief Executive Officer [Member] Chief Executive Officer Class of Stock [Domain] Class of Stock [Domain] Commitments and Contingencies Commitments and contingencies Commitments and Contingencies Disclosure [Abstract] Commitments and Contingencies Disclosure [Abstract] Commitments and Contingencies Disclosure [Line Items] Commitments and Contingencies Disclosure [Line Items] Commitments and Contingencies Disclosure [Line Items] Commitments and Contingencies Disclosure [Table] Commitments and Contingencies Disclosure [Table] Commitments and Contingencies Disclosure [Table] Commitments and Contingencies Disclosure [Text Block] COMMITMENTS AND CONTINGENCIES Common And Class B Common Stock [Member] Common And Class B Common Stock [Member] Common and Class B Common Stock Common Class B [Member] Class B Common Stock Common Stock Class B Common Stock And Preferred Stock [Member] Common Stock Class B Common Stock And Preferred Stock [Member] Common Stock, Class B Common Stock and Preferred Stock Common Stock, Dividends, Per Share, Declared Cash dividend Cash dividends declared and paid, common stock Common Stock [Member] Common Stock Common Stock, Par or Stated Value Per Share Common stock, par value Common Stock, Shares Authorized Common stock, shares authorized Common Stock, Shares, Outstanding Common stock, shares outstanding Common Stock, Value, Issued Common stock, $0.50 par value Common Stock, Voting Rights Amount of votes common stock is entitled Components Of Deferred Income Tax Assets And Liabilities [Table] Components Of Deferred Income Tax Assets And Liabilities [Table] Components Of Deferred Income Tax Assets And Liabilities [Table] Components Of Income Tax Expense Benefit [Line Items] Components Of Income Tax Expense Benefit [Line Items] Components Of Income Tax Expense Benefit [Line Items] Components Of Income Tax Expense Benefit [Table] Components Of Income Tax Expense Benefit [Table] Components Of Income Tax Expense Benefit [Table] Components of Net Deferred Tax Liabilities [Line Items] Components of Net Deferred Tax Liabilities [Line Items] Components of Net Deferred Tax Liabilities [Line Items] Tax effects allocated to each component of other comprehensive income loss. Components Of Other Comprehensive Income Loss [Line Items] Components Of Other Comprehensive Income Loss [Line Items] Comprehensive Income (Loss), Net of Tax, Attributable to Parent Comprehensive income attributable to Watsco, Inc. Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest Less: comprehensive income attributable to non-controlling interest Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest Comprehensive income Comprehensive Income (Loss) Note [Text Block] OTHER COMPREHENSIVE GAIN (LOSS) Computer Hardware And Software [Member] Computer Hardware And Software [Member] Computer Hardware and Software Concentration Risk Benchmark [Domain] Concentration Risk Benchmark [Domain] Concentration Risk Benchmark [Axis] Concentration Risk Benchmark [Axis] Concentration Risk Type [Axis] Concentration Risk Type [Axis] Concentration Risk Disclosure [Text Block] SUPPLIER CONCENTRATION Concentration Risk [Line Items] Concentration Risk [Line Items] Concentration Risk, Percentage Percentage of purchases from key suppliers Concentration Risk [Table] Concentration Risk [Table] Concentration Risk Type [Domain] Concentration Risk Type [Domain] Consolidated Entities [Axis] Consolidated Entities [Axis] Consolidated Entities [Domain] Consolidated Entities [Domain] The latest expiration date of foreign currency exchange contracts outstanding. Contracts Expiration Month and Year Contract expiring terms Contribution from non-controlling interest for investment in unconsolidated entity. Contribution From Non Controlling Interest For Investment In Unconsolidated Entity Investment in unconsolidated entity Class B common stock securities that may be converted to Common stock securities at any time on a one-for-one basis at the option of the shareholder. Convertible Class B Common Stock Outstanding Convertible Class B common stock outstanding Cost of Goods and Services Sold Cost of sales Cost of Goods, Total [Member] Cost of Goods, Total Cost of Sales, Vendor Allowances, Policy [Policy Text Block] Vendor Rebates Credit Facility [Axis] Credit Facility [Axis] Credit Facility [Domain] Credit Facility [Domain] Current Fiscal Year End Date Current Fiscal Year End Date Current Income Tax Expense (Benefit) Current Customary Fees for Legal Services [Member] Customary Fees for Legal Services [Member] Customary Fees for Legal Services Customary Fees for Remodeling of Corporate Headquarters [Member] Customary Fees for Remodeling of Corporate Headquarters [Member] Customary Payments for Remodeling of Corporate Headquarters Customer Relationships [Member] Customer Relationships Debt Disclosure [Abstract] Debt Disclosure [Abstract] Debt Disclosure [Text Block] DEBT Debt Instrument, Basis Spread on Variable Rate Basis spread Debt Instrument [Line Items] Debt Instrument [Line Items] Schedule of Long-term Debt Instruments [Table] Schedule of Long-term Debt Instruments [Table] Deferred Income Taxes and Other Liabilities, Noncurrent Deferred income taxes and other liabilities Deferred Income Tax Expense (Benefit) Deferred Deferred income tax (benefit) provision Deferred Tax Liabilities, Gross Total deferred tax liabilities Deferred Tax Assets, Gross Total deferred tax assets, gross Deferred Tax Assets, Inventory Capitalized inventory costs and inventory reserves Deferred Tax Assets, Net, Current Net current deferred tax assets and liabilities Deferred Tax Assets, Net of Valuation Allowance Total deferred tax assets Deferred Tax Assets, Operating Loss Carryforwards Net operating loss carryforwards Deferred Tax Assets, Other Other Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Share-based Compensation Cost Share-based compensation Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals, Allowance for Doubtful Accounts Allowance for doubtful accounts Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals, Self Insurance Self-insurance reserves Deferred Tax Assets, Valuation Allowance Valuation allowance Deferred Tax Assets, Valuation Allowance, Noncurrent Valuation allowance Deferred Tax Liabilities, Net Net deferred tax liabilities Deferred Tax Liabilities, Goodwill Deductible goodwill Deferred Tax Liabilities, Net, Noncurrent Net long-term deferred tax assets and liabilities Deferred Tax Liabilities, Other Other Deferred Tax Liabilities, Property, Plant and Equipment Depreciation Depreciation, Depletion and Amortization, Nonproduction Depreciation and amortization expense Depreciation, Depletion and Amortization Depreciation and amortization Derivative Asset, Current Derivative instruments, assets derivatives Derivative Contract [Domain] Derivative Contract [Domain] Derivative Instrument [Axis] Derivative Instrument [Axis] Derivative Instruments and Hedging Activities Disclosure [Abstract] Derivative Instruments and Hedging Activities Disclosure [Abstract] Derivative Instruments and Hedging Activities Disclosures [Line Items] Derivative Instruments and Hedging Activities Disclosures [Line Items] Derivative Instruments and Hedging Activities Disclosures [Table] Derivative Instruments and Hedging Activities Disclosures [Table] Derivative Instruments and Hedging Activities Disclosure [Text Block] DERIVATIVES Hedging Relationship [Axis] Hedging Relationship [Axis] Derivative Instruments, Gain (Loss) [Table] Derivative Instruments, Gain (Loss) [Table] Derivative Instruments, Gain (Loss) [Line Items] Derivative Instruments, Gain (Loss) [Line Items] Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net Gain (loss) reclassified from accumulated other comprehensive loss into earnings Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net Loss recorded in accumulated other comprehensive loss Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net (Loss) gain from foreign currency forward and option contracts not designated as hedging instruments Derivative Liability, Current Derivative instruments, liabilities derivatives Derivative maturity date range. Derivative Maturity Date Range Contract maturity period Derivative, Notional Amount Notional value of derivatives Derivatives, Fair Value [Line Items] Derivatives, Fair Value [Line Items] Derivatives, Policy [Policy Text Block] Derivative Instruments and Hedging Activity Designated as Hedging Instrument [Member] Derivatives Designated as Hedging Instruments Disclosure of Compensation Related Costs, Share-based Payments [Abstract] Disclosure of Compensation Related Costs, Share-based Payments [Abstract] Disclosure of Compensation Related Costs, Share-based Payments [Text Block] SHARE-BASED COMPENSATION AND BENEFIT PLANS Dividends Payable, Date to be Paid Dividends Payable, Date to be Paid Dividends, Common Stock, Cash Cash dividends declared and paid on Common and Class B common stock Dividends Payable, Date Declared Dividends, date declared Dividends Payable, Date of Record Dividends, date of record Document And Entity Information [Abstract] Document And Entity Information [Abstract] Document And Entity Information [Abstract] Document Fiscal Period Focus Document Fiscal Period Focus Document Fiscal Year Focus Document Fiscal Year Focus Document Information [Line Items] Document Information [Line Items] Document Information [Table] Document Information [Table] Document Period End Date Document Period End Date Document Type Document Type Domestic Tax Authority [Member] Federal Due to Related Parties, Current Amount payable to related party Earnings Per Share [Abstract] Earnings per share for Common and Class B common stock: Earnings Per Share, Basic Basic Basic earnings per share for Common and Class B common stock Earnings Per Share, Basic and Diluted [Abstract] Earnings per share for Common and Class B common stock: Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items] Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items] Earnings Per Share, Diluted Diluted Diluted earnings per share for Common and Class B common stock Earnings per share disclosures. Earnings Per Share [Line Items] Earnings Per Share [Line Items] Earnings Per Share, Policy [Policy Text Block] Earnings per Share Earnings Per Share [Table] Earnings Per Share [Table] Earnings Per Share [Table] Earnings Per Share [Text Block] EARNINGS PER SHARE Effective Income Tax Rate Reconciliation, Percent Effective income tax rate Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent U.S. federal statutory rate Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent Deferred tax impact of enacted tax rate changes Effective income tax rate reconciliation controlling interest. Effective Income Tax Rate Reconciliation Controlling Interest Effective income tax rate attributable to Watsco, Inc. Effective Income Tax Rate Reconciliation, Foreign Income Tax Rate Differential, Percent Tax effects on foreign income Effective Income Tax Rate Reconciliation, Noncontrolling Interest Income (Loss), Percent Taxes attributable to non-controlling interest Effective Income Tax Rate Reconciliation, Nondeductible Expense, Share-based Compensation Cost, Percent Excess tax benefits from share-based compensation Effective Income Tax Rate Reconciliation, Repatriation of Foreign Earnings, Percent Repatriation transition tax Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent State income taxes, net of federal benefit and other Effective income tax rate reconciliation tax credits and other. Effective Income Tax Rate Reconciliation Tax Credits and Other Tax credits and other Effect of Exchange Rate on Cash and Cash Equivalents Effect of foreign exchange rate changes on cash and cash equivalents Employee [Member] Employee [Member] Employee Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized Unrecognized share-based compensation expense Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition Weighted-average period for recognition of share-based compensation expense, in years Employee Stock [Member] Employee Stock Purchase Plan Employee Stock Option [Member] Stock Options Entity Central Index Key Entity Central Index Key Entity Common Stock, Shares Outstanding Entity Common Stock, Shares Outstanding Entity Current Reporting Status Entity Current Reporting Status Entity [Domain] Entity [Domain] Entity Filer Category Entity Filer Category Entity Public Float Entity Public Float Entity Registrant Name Entity Registrant Name Entity Voluntary Filers Entity Voluntary Filers Entity Well-known Seasoned Issuer Entity Well-known Seasoned Issuer