Revenue from Contracts with Customers - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue
Revenue	$ 2,856,659	$ 2,615,277
Percentage of revenue from contract with customer (as a percent)	100.00%	100.00%
Industrial
Disaggregation of Revenue
Revenue	$ 1,112,075	$ 886,668
Percentage of revenue from contract with customer (as a percent)	38.90%	33.90%
Education
Disaggregation of Revenue
Revenue	$ 487,922	$ 412,318
Percentage of revenue from contract with customer (as a percent)	17.10%	15.80%
Office Buildings
Disaggregation of Revenue
Revenue	$ 319,426	$ 348,640
Percentage of revenue from contract with customer (as a percent)	11.20%	13.30%
Healthcare
Disaggregation of Revenue
Revenue	$ 371,105	$ 358,155
Percentage of revenue from contract with customer (as a percent)	13.00%	13.70%
Government
Disaggregation of Revenue
Revenue	$ 163,717	$ 162,507
Percentage of revenue from contract with customer (as a percent)	5.70%	6.20%
Retail, Restaurants and Entertainment
Disaggregation of Revenue
Revenue	$ 239,541	$ 248,083
Percentage of revenue from contract with customer (as a percent)	8.40%	9.50%
Multi-Family and Residential
Disaggregation of Revenue
Revenue	$ 86,799	$ 104,693
Percentage of revenue from contract with customer (as a percent)	3.00%	4.00%
Other
Disaggregation of Revenue
Revenue	$ 76,074	$ 94,213
Percentage of revenue from contract with customer (as a percent)	2.70%	3.60%
New Construction
Disaggregation of Revenue
Revenue	$ 1,333,739	$ 1,201,122
Percentage of revenue from contract with customer (as a percent)	46.70%	45.90%
Existing Building Construction
Disaggregation of Revenue
Revenue	$ 910,807	$ 793,159
Percentage of revenue from contract with customer (as a percent)	31.90%	30.30%
Service Projects
Disaggregation of Revenue
Revenue	$ 241,402	$ 231,228
Percentage of revenue from contract with customer (as a percent)	8.40%	8.90%
Service Calls, Maintenance and Monitoring
Disaggregation of Revenue
Revenue	$ 370,711	$ 389,768
Percentage of revenue from contract with customer (as a percent)	13.00%	14.90%
Mechanical Services
Disaggregation of Revenue
Revenue	$ 2,413,016	$ 2,251,560
Percentage of revenue from contract with customer (as a percent)	84.50%	86.10%
Electrical Services
Disaggregation of Revenue
Revenue	$ 443,643	$ 363,717
Percentage of revenue from contract with customer (as a percent)	15.50%	13.90%
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Disaggregation of Revenue
Revenue			$ 2,182,879
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			$ 596,557
Percentage of revenue from contract with customer (as a percent)			27.30%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Education
Disaggregation of Revenue
Revenue			$ 391,937
Percentage of revenue from contract with customer (as a percent)			18.00%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Office Buildings
Disaggregation of Revenue
Revenue			$ 288,090
Percentage of revenue from contract with customer (as a percent)			13.20%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Healthcare
Disaggregation of Revenue
Revenue			$ 319,958
Percentage of revenue from contract with customer (as a percent)			14.70%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Government
Disaggregation of Revenue
Revenue			$ 143,958
Percentage of revenue from contract with customer (as a percent)			6.60%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Retail, Restaurants and Entertainment
Disaggregation of Revenue
Revenue			$ 225,348
Percentage of revenue from contract with customer (as a percent)			10.30%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Multi-Family and Residential
Disaggregation of Revenue
Revenue			$ 136,075
Percentage of revenue from contract with customer (as a percent)			6.20%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Other
Disaggregation of Revenue
Revenue			$ 80,956
Percentage of revenue from contract with customer (as a percent)			3.70%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | New Construction
Disaggregation of Revenue
Revenue			$ 829,978
Percentage of revenue from contract with customer (as a percent)			38.00%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Existing Building Construction
Disaggregation of Revenue
Revenue			$ 796,946
Percentage of revenue from contract with customer (as a percent)			36.50%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Service Projects
Disaggregation of Revenue
Revenue			$ 206,506
Percentage of revenue from contract with customer (as a percent)			9.50%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Service Calls, Maintenance and Monitoring
Disaggregation of Revenue
Revenue			$ 349,449
Percentage of revenue from contract with customer (as a percent)			16.00%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Mechanical Services
Disaggregation of Revenue
Revenue			$ 2,176,223
Percentage of revenue from contract with customer (as a percent)			99.70%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Electrical Services
Disaggregation of Revenue
Revenue			$ 6,656
Percentage of revenue from contract with customer (as a percent)			0.30%