Summary of Significant Accounting Policies - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Revenue
Revenue, Performance Obligation, Description of Payment Terms			We typically invoice our customers with payment terms of net due in 30 days
Receivable payment terms (in days)			30 days
Revenue	$ 706,918	$ 594,536	$ 1,895,693	$ 1,594,520
Percentage of revenue from contract with customer (as a percent)	100.00%	100.00%	100.00%	100.00%
Industrial
Disaggregation of Revenue
Revenue	$ 222,156	$ 184,174	$ 588,818	$ 398,351
Percentage of revenue from contract with customer (as a percent)	31.40%	31.00%	31.10%	25.00%
Education
Disaggregation of Revenue
Revenue	$ 125,836	$ 115,202	$ 292,799	$ 312,852
Percentage of revenue from contract with customer (as a percent)	17.80%	19.40%	15.40%	19.60%
Office Buildings
Disaggregation of Revenue
Revenue	$ 83,271	$ 75,871	$ 254,966	$ 224,300
Percentage of revenue from contract with customer (as a percent)	11.80%	12.80%	13.50%	14.10%
Healthcare
Disaggregation of Revenue
Revenue	$ 126,909	$ 79,424	$ 306,810	$ 214,537
Percentage of revenue from contract with customer (as a percent)	18.00%	13.30%	16.20%	13.40%
Government
Disaggregation of Revenue
Revenue	$ 41,502	$ 32,633	$ 118,224	$ 106,065
Percentage of revenue from contract with customer (as a percent)	5.90%	5.50%	6.20%	6.60%
Retail, Restaurants and Entertainment
Disaggregation of Revenue
Revenue	$ 67,141	$ 60,908	$ 184,618	$ 169,899
Percentage of revenue from contract with customer (as a percent)	9.50%	10.20%	9.70%	10.70%
Multi-Family and Residential
Disaggregation of Revenue
Revenue	$ 24,121	$ 33,903	$ 83,417	$ 102,995
Percentage of revenue from contract with customer (as a percent)	3.40%	5.70%	4.40%	6.50%
Other
Disaggregation of Revenue
Revenue	$ 15,982	$ 12,421	$ 66,041	$ 65,521
Percentage of revenue from contract with customer (as a percent)	2.20%	2.10%	3.50%	4.10%
New Construction
Disaggregation of Revenue
Revenue	$ 312,279	$ 227,715	$ 827,718	$ 606,208
Percentage of revenue from contract with customer (as a percent)	44.20%	38.30%	43.70%	38.00%
Existing Building Construction
Disaggregation of Revenue
Revenue	$ 224,125	$ 216,010	$ 605,819	$ 572,808
Percentage of revenue from contract with customer (as a percent)	31.60%	36.30%	31.90%	35.90%
Service Projects
Disaggregation of Revenue
Revenue	$ 62,668	$ 56,557	$ 171,860	$ 150,890
Percentage of revenue from contract with customer (as a percent)	8.90%	9.50%	9.10%	9.50%
Service Calls, Maintenance and Monitoring
Disaggregation of Revenue
Revenue	$ 107,846	$ 94,254	$ 290,296	$ 264,614
Percentage of revenue from contract with customer (as a percent)	15.30%	15.90%	15.30%	16.60%
HVAC and Plumbing
Disaggregation of Revenue
Revenue	$ 545,088	$ 540,733	$ 1,531,331	$ 1,448,761
Percentage of revenue from contract with customer (as a percent)	77.10%	91.00%	80.80%	90.90%
Electrical Services
Disaggregation of Revenue
Revenue	$ 116,902	$ 1,611	$ 218,061	$ 1,611
Percentage of revenue from contract with customer (as a percent)	16.50%	0.30%	11.50%	0.10%
Building Automation Control Systems
Disaggregation of Revenue
Revenue	$ 24,540	$ 25,822	$ 73,816	$ 72,128
Percentage of revenue from contract with customer (as a percent)	3.50%	4.30%	3.90%	4.50%
Other.
Disaggregation of Revenue
Revenue	$ 20,388	$ 26,370	$ 72,485	$ 72,020
Percentage of revenue from contract with customer (as a percent)	2.90%	4.40%	3.80%	4.50%
Minimum
Disaggregation of Revenue
Payments received term (in days)			30 days
Maximum
Disaggregation of Revenue
Payments received term (in days)			90 days