Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2019	Oct. 18, 2019
Document and Entity Information
Document Type	10-Q
Document Quarterly Report	true
Document Transition Report	false
Document Period End Date	Sep. 30, 2019
Entity Registrant Name	COMFORT SYSTEMS USA, INC.
Entity File Number	1-13011
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	76-0526487
Entity Address, Address Line One	675 Bering Drive
Entity Address, Address Line Two	Suite 400
Entity Address, City or Town	Houston
Entity Address, State or Province	TX
Entity Address, Postal Zip Code	77057
City Area Code	713
Local Phone Number	830-9600
Entity Central Index Key	0001035983
Title of 12(b) Security	Common Stock, $0.01 par value
Trading Symbol	FIX
Security Exchange Name	NYSE
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q3
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		36,675,927