v3.8.0.1
Document and Entity Information - shares
3 Months Ended
Mar. 31, 2018
May 04, 2018
Document Information [Line Items]    
Document Type 10-Q  
Amendment Flag false  
Document Period End Date Mar. 31, 2018  
Document Fiscal Year Focus 2018  
Document Fiscal Period Focus Q1  
Trading Symbol WSO  
Entity Registrant Name WATSCO INC  
Entity Central Index Key 0000105016  
Current Fiscal Year End Date --12-31  
Entity Filer Category Large Accelerated Filer  
Common Stock    
Document Information [Line Items]    
Entity Common Stock, Shares Outstanding   32,042,811
Class B Common Stock    
Document Information [Line Items]    
Entity Common Stock, Shares Outstanding   5,316,683