Summary of Significant Accounting Policies - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
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	$ 2,182,879
Percentage of revenue from contract with customer (as a percent)	100.00%
Industrial
Disaggregation of Revenue
Revenue	$ 596,557
Percentage of revenue from contract with customer (as a percent)	27.30%
Education
Disaggregation of Revenue
Revenue	$ 391,937
Percentage of revenue from contract with customer (as a percent)	18.00%
Office Buildings
Disaggregation of Revenue
Revenue	$ 288,090
Percentage of revenue from contract with customer (as a percent)	13.20%
Healthcare
Disaggregation of Revenue
Revenue	$ 319,958
Percentage of revenue from contract with customer (as a percent)	14.70%
Government
Disaggregation of Revenue
Revenue	$ 143,958
Percentage of revenue from contract with customer (as a percent)	6.60%
Retail, Restaurants and Entertainment
Disaggregation of Revenue
Revenue	$ 225,348
Percentage of revenue from contract with customer (as a percent)	10.30%
Multi-Family and Residential
Disaggregation of Revenue
Revenue	$ 136,075
Percentage of revenue from contract with customer (as a percent)	6.20%
Other
Disaggregation of Revenue
Revenue	$ 80,956
Percentage of revenue from contract with customer (as a percent)	3.70%
New Construction
Disaggregation of Revenue
Revenue	$ 829,978
Percentage of revenue from contract with customer (as a percent)	38.00%
Existing Building Construction
Disaggregation of Revenue
Revenue	$ 796,946
Percentage of revenue from contract with customer (as a percent)	36.50%
Service Projects
Disaggregation of Revenue
Revenue	$ 206,506
Percentage of revenue from contract with customer (as a percent)	9.50%
Service Calls, Maintenance and Monitoring
Disaggregation of Revenue
Revenue	$ 349,448
Percentage of revenue from contract with customer (as a percent)	16.00%
HVAC and Plumbing
Disaggregation of Revenue
Revenue	$ 1,976,374
Percentage of revenue from contract with customer (as a percent)	90.50%
Building Automation Control Systems
Disaggregation of Revenue
Revenue	$ 95,093
Percentage of revenue from contract with customer (as a percent)	4.40%
Other
Disaggregation of Revenue
Revenue	$ 111,412
Percentage of revenue from contract with customer (as a percent)	5.10%
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Disaggregation of Revenue
Revenue		$ 1,787,922	$ 1,634,340
Percentage of revenue from contract with customer (as a percent)		100.00%	100.00%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Industrial
Disaggregation of Revenue
Revenue		$ 395,362	$ 378,499
Percentage of revenue from contract with customer (as a percent)		22.10%	23.20%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Education
Disaggregation of Revenue
Revenue		$ 351,808	$ 307,195
Percentage of revenue from contract with customer (as a percent)		19.70%	18.80%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Office Buildings
Disaggregation of Revenue
Revenue		$ 248,604	$ 202,187
Percentage of revenue from contract with customer (as a percent)		13.90%	12.40%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Healthcare
Disaggregation of Revenue
Revenue		$ 224,643	$ 197,797
Percentage of revenue from contract with customer (as a percent)		12.60%	12.10%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Government
Disaggregation of Revenue
Revenue		$ 140,843	$ 160,414
Percentage of revenue from contract with customer (as a percent)		7.90%	9.80%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Retail, Restaurants and Entertainment
Disaggregation of Revenue
Revenue		$ 223,593	$ 196,094
Percentage of revenue from contract with customer (as a percent)		12.50%	12.00%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Multi-Family and Residential
Disaggregation of Revenue
Revenue		$ 116,844	$ 122,766
Percentage of revenue from contract with customer (as a percent)		6.50%	7.50%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Other
Disaggregation of Revenue
Revenue		$ 86,225	$ 69,388
Percentage of revenue from contract with customer (as a percent)		4.80%	4.20%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | New Construction
Disaggregation of Revenue
Revenue		$ 684,687	$ 653,488
Percentage of revenue from contract with customer (as a percent)		38.30%	40.00%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Existing Building Construction
Disaggregation of Revenue
Revenue		$ 580,737	$ 514,437
Percentage of revenue from contract with customer (as a percent)		32.40%	31.50%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Service Projects
Disaggregation of Revenue
Revenue		$ 197,703	$ 178,455
Percentage of revenue from contract with customer (as a percent)		11.10%	10.90%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Service Calls, Maintenance and Monitoring
Disaggregation of Revenue
Revenue		$ 324,795	$ 287,960
Percentage of revenue from contract with customer (as a percent)		18.20%	17.60%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | HVAC and Plumbing
Disaggregation of Revenue
Revenue		$ 1,615,468	$ 1,462,980
Percentage of revenue from contract with customer (as a percent)		90.40%	89.50%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Building Automation Control Systems
Disaggregation of Revenue
Revenue		$ 94,041	$ 90,461
Percentage of revenue from contract with customer (as a percent)		5.30%	5.50%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Other
Disaggregation of Revenue
Revenue		$ 78,413	$ 80,899
Percentage of revenue from contract with customer (as a percent)		4.40%	4.90%
Minimum
Disaggregation of Revenue
Payments received term (in days)	30 days
Maximum
Disaggregation of Revenue
Payments received term (in days)	90 days