Summary of Significant Accounting Policies - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue
Revenue	$ 2,615,277	$ 2,182,879
Percentage of revenue from contract with customer (as a percent)	100.00%	100.00%
Industrial
Disaggregation of Revenue
Revenue	$ 886,668	$ 596,557
Percentage of revenue from contract with customer (as a percent)	33.90%	27.30%
Education
Disaggregation of Revenue
Revenue	$ 412,318	$ 391,937
Percentage of revenue from contract with customer (as a percent)	15.80%	18.00%
Office Buildings
Disaggregation of Revenue
Revenue	$ 348,640	$ 288,090
Percentage of revenue from contract with customer (as a percent)	13.30%	13.20%
Healthcare
Disaggregation of Revenue
Revenue	$ 358,155	$ 319,958
Percentage of revenue from contract with customer (as a percent)	13.70%	14.70%
Government
Disaggregation of Revenue
Revenue	$ 162,507	$ 143,958
Percentage of revenue from contract with customer (as a percent)	6.20%	6.60%
Retail, Restaurants and Entertainment
Disaggregation of Revenue
Revenue	$ 248,083	$ 225,348
Percentage of revenue from contract with customer (as a percent)	9.50%	10.30%
Multi-Family and Residential
Disaggregation of Revenue
Revenue	$ 104,693	$ 136,075
Percentage of revenue from contract with customer (as a percent)	4.00%	6.20%
Other
Disaggregation of Revenue
Revenue	$ 94,213	$ 80,956
Percentage of revenue from contract with customer (as a percent)	3.60%	3.70%
New Construction
Disaggregation of Revenue
Revenue	$ 1,201,122	$ 829,978
Percentage of revenue from contract with customer (as a percent)	45.90%	38.00%
Existing Building Construction
Disaggregation of Revenue
Revenue	$ 793,159	$ 796,946
Percentage of revenue from contract with customer (as a percent)	30.30%	36.50%
Service Projects
Disaggregation of Revenue
Revenue	$ 231,228	$ 206,506
Percentage of revenue from contract with customer (as a percent)	8.90%	9.50%
Service Calls, Maintenance and Monitoring
Disaggregation of Revenue
Revenue	$ 389,768	$ 349,449
Percentage of revenue from contract with customer (as a percent)	14.90%	16.00%
Mechanical Services
Disaggregation of Revenue
Revenue	$ 2,251,560	$ 2,176,223
Percentage of revenue from contract with customer (as a percent)	86.10%	99.70%
Electrical Services
Disaggregation of Revenue
Revenue	$ 363,717	$ 6,656
Percentage of revenue from contract with customer (as a percent)	13.90%	0.30%
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Disaggregation of Revenue
Revenue			$ 1,787,922
Percentage of revenue from contract with customer (as a percent)			100.00%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Industrial
Disaggregation of Revenue
Revenue			$ 395,362
Percentage of revenue from contract with customer (as a percent)			22.10%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Education
Disaggregation of Revenue
Revenue			$ 351,808
Percentage of revenue from contract with customer (as a percent)			19.70%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Office Buildings
Disaggregation of Revenue
Revenue			$ 248,604
Percentage of revenue from contract with customer (as a percent)			13.90%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Healthcare
Disaggregation of Revenue
Revenue			$ 224,643
Percentage of revenue from contract with customer (as a percent)			12.60%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Government
Disaggregation of Revenue
Revenue			$ 140,843
Percentage of revenue from contract with customer (as a percent)			7.90%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Retail, Restaurants and Entertainment
Disaggregation of Revenue
Revenue			$ 223,593
Percentage of revenue from contract with customer (as a percent)			12.50%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Multi-Family and Residential
Disaggregation of Revenue
Revenue			$ 116,844
Percentage of revenue from contract with customer (as a percent)			6.50%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Other
Disaggregation of Revenue
Revenue			$ 86,225
Percentage of revenue from contract with customer (as a percent)			4.80%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | New Construction
Disaggregation of Revenue
Revenue			$ 684,687
Percentage of revenue from contract with customer (as a percent)			38.30%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Existing Building Construction
Disaggregation of Revenue
Revenue			$ 580,737
Percentage of revenue from contract with customer (as a percent)			32.40%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Service Projects
Disaggregation of Revenue
Revenue			$ 197,703
Percentage of revenue from contract with customer (as a percent)			11.10%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Service Calls, Maintenance and Monitoring
Disaggregation of Revenue
Revenue			$ 324,795
Percentage of revenue from contract with customer (as a percent)			18.20%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Mechanical Services
Disaggregation of Revenue
Revenue			$ 1,787,922
Percentage of revenue from contract with customer (as a percent)			100.00%