Summary of Significant Accounting Policies - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Revenue
Revenue	$ 743,468	$ 650,302	$ 1,443,599	$ 1,188,775
Percentage of revenue from contract with customer (as a percent)	100.00%	100.00%	100.00%	100.00%
Industrial
Disaggregation of Revenue
Revenue	$ 300,870	$ 198,002	$ 576,068	$ 366,662
Percentage of revenue from contract with customer (as a percent)	40.50%	30.50%	39.90%	30.80%
Education
Disaggregation of Revenue
Revenue	$ 130,004	$ 100,220	$ 239,588	$ 166,963
Percentage of revenue from contract with customer (as a percent)	17.50%	15.40%	16.60%	14.10%
Office Buildings
Disaggregation of Revenue
Revenue	$ 75,594	$ 105,483	$ 151,166	$ 171,695
Percentage of revenue from contract with customer (as a percent)	10.20%	16.20%	10.50%	14.40%
Healthcare
Disaggregation of Revenue
Revenue	$ 96,050	$ 87,878	$ 195,309	$ 179,901
Percentage of revenue from contract with customer (as a percent)	12.90%	13.50%	13.50%	15.10%
Government
Disaggregation of Revenue
Revenue	$ 39,832	$ 44,443	$ 78,813	$ 76,722
Percentage of revenue from contract with customer (as a percent)	5.40%	6.80%	5.50%	6.50%
Retail, Restaurants and Entertainment
Disaggregation of Revenue
Revenue	$ 64,628	$ 58,086	$ 125,831	$ 117,477
Percentage of revenue from contract with customer (as a percent)	8.70%	8.90%	8.70%	9.90%
Multi-Family and Residential
Disaggregation of Revenue
Revenue	$ 20,555	$ 29,061	$ 39,286	$ 59,296
Percentage of revenue from contract with customer (as a percent)	2.80%	4.50%	2.70%	5.00%
Other
Disaggregation of Revenue
Revenue	$ 15,935	$ 27,129	$ 37,538	$ 50,059
Percentage of revenue from contract with customer (as a percent)	2.00%	4.20%	2.60%	4.20%
New Construction
Disaggregation of Revenue
Revenue	$ 377,433	$ 291,479	$ 724,833	$ 515,439
Percentage of revenue from contract with customer (as a percent)	50.80%	44.80%	50.20%	43.40%
Existing Building Construction
Disaggregation of Revenue
Revenue	$ 225,103	$ 199,398	$ 432,269	$ 381,694
Percentage of revenue from contract with customer (as a percent)	30.30%	30.70%	29.90%	32.10%
Service Projects
Disaggregation of Revenue
Revenue	$ 58,378	$ 58,808	$ 110,026	$ 109,192
Percentage of revenue from contract with customer (as a percent)	7.80%	9.00%	7.60%	9.20%
Service Calls, Maintenance and Monitoring
Disaggregation of Revenue
Revenue	$ 82,554	$ 100,617	$ 176,471	$ 182,450
Percentage of revenue from contract with customer (as a percent)	11.10%	15.50%	12.30%	15.30%
Mechanical Services
Disaggregation of Revenue
Revenue	$ 622,700	$ 553,031	$ 1,188,164	$ 1,087,616
Percentage of revenue from contract with customer (as a percent)	83.80%	85.00%	82.30%	91.50%
Electrical Services
Disaggregation of Revenue
Revenue	$ 120,768	$ 97,271	$ 255,435	$ 101,159
Percentage of revenue from contract with customer (as a percent)	16.20%	15.00%	17.70%	8.50%