Schedule of Basic and Diluted Earnings per Common Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income attributable to Watsco, Inc. shareholders	$ 89,957	$ 73,756	$ 124,176	$ 99,937
Less: distributed and undistributed earnings allocated to non-vested restricted common stock - Basic	7,379	6,189	10,147	8,376
Earnings allocated to Watsco, Inc. shareholders - Basic	$ 82,578	$ 67,567	$ 114,029	$ 91,561
Weighted-average common shares outstanding - Basic	34,309,885	32,682,474	34,282,261	32,662,653
Basic earnings per share for Common and Class B common stock	$ 2.41	$ 2.07	$ 3.33	$ 2.80
Net income attributable to Watsco, Inc. shareholders	$ 89,957	$ 73,756	$ 124,176	$ 99,937
Less: distributed and undistributed earnings allocated to non-vested restricted common stock - Diluted	7,374	6,186	10,144	8,374
Earnings allocated to Watsco, Inc. shareholders - Diluted	$ 82,583	$ 67,570	$ 114,032	$ 91,563
Weighted-average common shares outstanding - Basic	34,309,885	32,682,474	34,282,261	32,662,653
Effect of dilutive stock options	69,310	26,172	67,554	31,653
Weighted-average common shares outstanding - Diluted	34,379,195	32,708,646	34,349,815	32,694,306
Diluted earnings per share for Common and Class B common stock	$ 2.40	$ 2.07	$ 3.32	$ 2.80
Anti-dilutive stock options not included above	40,797	63,467	24,044	27,787
Common Stock
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Earnings allocated to Watsco, Inc. shareholders - Basic	$ 76,321	$ 61,966	$ 105,383	$ 83,966
Class B Common Stock
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Earnings allocated to Watsco, Inc. shareholders - Basic	$ 6,257	$ 5,601	$ 8,646	$ 7,595