| Document and Entity Information | Jul. 23, 2020 |
| Document And Entity Information [Line Items] | |
| Entity Registrant Name | WATSCO INC |
| Amendment Flag | false |
| Entity Central Index Key | 0000105016 |
| Document Type | 8-K |
| Document Period End Date | Jul. 23, 2020 |
| Entity Incorporation State Country Code | FL |
| Entity File Number | 1-5581 |
| Entity Tax Identification Number | 59-0778222 |
| Entity Address, Address Line One | 2665 South Bayshore Drive |
| Entity Address, Address Line Two | Suite 901 |
| Entity Address, City or Town | Miami |
| Entity Address, State or Province | FL |
| Entity Address, Postal Zip Code | 33133 |
| City Area Code | (305) |
| Local Phone Number | 714-4100 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Common Stock [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | Common stock, $0.50 par value |
| Trading Symbol | WSO |
| Security Exchange Name | NYSE |
| Common Class B [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | Class B common stock, $0.50 par value |
| Trading Symbol | WSOB |
| Security Exchange Name | NYSE |
