Summary of Significant Accounting Policies - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disaggregation of Revenue
Receivable payment terms (in days)	30 days
Revenue	$ 464,941
Percentage of revenue from contract with customer (as a percent)	100.00%
Industrial
Disaggregation of Revenue
Revenue	$ 100,100
Percentage of revenue from contract with customer (as a percent)	22.00%
Education
Disaggregation of Revenue
Revenue	$ 88,203
Percentage of revenue from contract with customer (as a percent)	19.00%
Office Buildings
Disaggregation of Revenue
Revenue	$ 69,120
Percentage of revenue from contract with customer (as a percent)	15.00%
Healthcare
Disaggregation of Revenue
Revenue	$ 63,183
Percentage of revenue from contract with customer (as a percent)	14.00%
Government
Disaggregation of Revenue
Revenue	$ 36,147
Percentage of revenue from contract with customer (as a percent)	8.00%
Retail, Restaurants and Entertainment
Disaggregation of Revenue
Revenue	$ 52,787
Percentage of revenue from contract with customer (as a percent)	11.00%
Multi-Family and Residential
Disaggregation of Revenue
Revenue	$ 33,052
Percentage of revenue from contract with customer (as a percent)	7.00%
Other
Disaggregation of Revenue
Revenue	$ 22,349
Percentage of revenue from contract with customer (as a percent)	4.00%
New Construction
Disaggregation of Revenue
Revenue	$ 183,437
Percentage of revenue from contract with customer (as a percent)	39.00%
Existing Building Construction
Disaggregation of Revenue
Revenue	$ 155,197
Percentage of revenue from contract with customer (as a percent)	33.00%
Small Projects
Disaggregation of Revenue
Revenue	$ 40,175
Percentage of revenue from contract with customer (as a percent)	9.00%
Service Calls, Maintenance and Monitoring
Disaggregation of Revenue
Revenue	$ 86,132
Percentage of revenue from contract with customer (as a percent)	19.00%
HVAC and Plumbing
Disaggregation of Revenue
Revenue	$ 424,017
Percentage of revenue from contract with customer (as a percent)	91.00%
Building Automation Control Systems
Disaggregation of Revenue
Revenue	$ 20,045
Percentage of revenue from contract with customer (as a percent)	4.00%
Other
Disaggregation of Revenue
Revenue	$ 20,879
Percentage of revenue from contract with customer (as a percent)	5.00%
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Disaggregation of Revenue
Revenue		$ 380,588
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		$ 87,614
Percentage of revenue from contract with customer (as a percent)		23.00%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Education
Disaggregation of Revenue
Revenue		$ 61,588
Percentage of revenue from contract with customer (as a percent)		16.00%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Office Buildings
Disaggregation of Revenue
Revenue		$ 52,434
Percentage of revenue from contract with customer (as a percent)		14.00%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Healthcare
Disaggregation of Revenue
Revenue		$ 51,767
Percentage of revenue from contract with customer (as a percent)		14.00%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Government
Disaggregation of Revenue
Revenue		$ 36,236
Percentage of revenue from contract with customer (as a percent)		10.00%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Retail, Restaurants and Entertainment
Disaggregation of Revenue
Revenue		$ 45,225
Percentage of revenue from contract with customer (as a percent)		12.00%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Multi-Family and Residential
Disaggregation of Revenue
Revenue		$ 26,870
Percentage of revenue from contract with customer (as a percent)		7.00%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Other
Disaggregation of Revenue
Revenue		$ 18,854
Percentage of revenue from contract with customer (as a percent)		4.00%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | New Construction
Disaggregation of Revenue
Revenue		$ 148,511
Percentage of revenue from contract with customer (as a percent)		39.00%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Existing Building Construction
Disaggregation of Revenue
Revenue		$ 118,266
Percentage of revenue from contract with customer (as a percent)		31.00%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Small Projects
Disaggregation of Revenue
Revenue		$ 42,609
Percentage of revenue from contract with customer (as a percent)		11.00%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Service Calls, Maintenance and Monitoring
Disaggregation of Revenue
Revenue		$ 71,202
Percentage of revenue from contract with customer (as a percent)		19.00%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | HVAC and Plumbing
Disaggregation of Revenue
Revenue		$ 338,723
Percentage of revenue from contract with customer (as a percent)		89.00%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Building Automation Control Systems
Disaggregation of Revenue
Revenue		$ 22,835
Percentage of revenue from contract with customer (as a percent)		6.00%
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Other
Disaggregation of Revenue
Revenue		$ 19,030
Percentage of revenue from contract with customer (as a percent)		5.00%
Minimum
Disaggregation of Revenue
Payments received term (in days)	30 days
Maximum
Disaggregation of Revenue
Payments received term (in days)	90 days