Summary of Significant Accounting Policies - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 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	$ 650,302	$ 535,043	$ 1,188,775	$ 999,984
Percentage of revenue from contract with customer (as a percent)	100.00%	100.00%	100.00%	100.00%
Industrial
Disaggregation of Revenue
Revenue	$ 198,002	$ 114,077	$ 366,662	$ 214,177
Percentage of revenue from contract with customer (as a percent)	30.50%	21.30%	30.80%	21.40%
Education
Disaggregation of Revenue
Revenue	$ 100,220	$ 109,447	$ 166,963	$ 197,650
Percentage of revenue from contract with customer (as a percent)	15.40%	20.50%	14.10%	19.80%
Office Buildings
Disaggregation of Revenue
Revenue	$ 105,483	$ 79,309	$ 171,695	$ 148,429
Percentage of revenue from contract with customer (as a percent)	16.20%	14.80%	14.40%	14.90%
Healthcare
Disaggregation of Revenue
Revenue	$ 87,878	$ 71,930	$ 179,901	$ 135,113
Percentage of revenue from contract with customer (as a percent)	13.50%	13.40%	15.10%	13.50%
Government
Disaggregation of Revenue
Revenue	$ 44,443	$ 37,285	$ 76,722	$ 73,432
Percentage of revenue from contract with customer (as a percent)	6.80%	7.00%	6.50%	7.30%
Retail, Restaurants and Entertainment
Disaggregation of Revenue
Revenue	$ 58,086	$ 56,204	$ 117,477	$ 108,991
Percentage of revenue from contract with customer (as a percent)	8.90%	10.50%	9.90%	10.90%
Multi-Family and Residential
Disaggregation of Revenue
Revenue	$ 29,061	$ 36,040	$ 59,296	$ 69,092
Percentage of revenue from contract with customer (as a percent)	4.50%	6.70%	5.00%	6.90%
Other
Disaggregation of Revenue
Revenue	$ 27,129	$ 30,751	$ 50,059	$ 53,100
Percentage of revenue from contract with customer (as a percent)	4.20%	5.80%	4.20%	5.30%
New Construction
Disaggregation of Revenue
Revenue	$ 291,479	$ 193,211	$ 515,439	$ 378,493
Percentage of revenue from contract with customer (as a percent)	44.80%	36.10%	43.40%	37.90%
Existing Building Construction
Disaggregation of Revenue
Revenue	$ 199,398	$ 200,040	$ 381,694	$ 356,798
Percentage of revenue from contract with customer (as a percent)	30.70%	37.40%	32.10%	35.70%
Service Projects
Disaggregation of Revenue
Revenue	$ 58,808	$ 51,900	$ 109,192	$ 94,333
Percentage of revenue from contract with customer (as a percent)	9.00%	9.70%	9.20%	9.40%
Service Calls, Maintenance and Monitoring
Disaggregation of Revenue
Revenue	$ 100,617	$ 89,892	$ 182,450	$ 170,360
Percentage of revenue from contract with customer (as a percent)	15.50%	16.80%	15.30%	17.00%
HVAC and Plumbing
Disaggregation of Revenue
Revenue	$ 504,253	$ 484,011	$ 986,243	$ 908,028
Percentage of revenue from contract with customer (as a percent)	77.50%	90.50%	83.00%	90.80%
Electrical Services
Disaggregation of Revenue
Revenue	$ 97,271		$ 101,159
Percentage of revenue from contract with customer (as a percent)	15.00%		8.50%
Building Automation Control Systems
Disaggregation of Revenue
Revenue	$ 20,262	$ 26,261	$ 49,276	$ 46,306
Percentage of revenue from contract with customer (as a percent)	3.10%	4.90%	4.10%	4.60%
Other.
Disaggregation of Revenue
Revenue	$ 28,516	$ 24,771	$ 52,097	$ 45,650
Percentage of revenue from contract with customer (as a percent)	4.40%	4.60%	4.40%	4.60%
Minimum
Disaggregation of Revenue
Payments received term (in days)			30 days
Maximum
Disaggregation of Revenue
Payments received term (in days)			90 days