Summary of Significant Accounting Policies - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 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	$ 538,473	$ 464,941
Percentage of revenue from contract with customer (as a percent)	100.00%	100.00%
Industrial
Disaggregation of Revenue
Revenue	$ 168,660	$ 100,100
Percentage of revenue from contract with customer (as a percent)	31.30%	21.50%
Education
Disaggregation of Revenue
Revenue	$ 66,743	$ 88,203
Percentage of revenue from contract with customer (as a percent)	12.40%	19.00%
Office Buildings
Disaggregation of Revenue
Revenue	$ 66,212	$ 69,120
Percentage of revenue from contract with customer (as a percent)	12.30%	14.90%
Healthcare
Disaggregation of Revenue
Revenue	$ 92,023	$ 63,183
Percentage of revenue from contract with customer (as a percent)	17.10%	13.60%
Government
Disaggregation of Revenue
Revenue	$ 32,279	$ 36,147
Percentage of revenue from contract with customer (as a percent)	6.00%	7.80%
Retail, Restaurants and Entertainment
Disaggregation of Revenue
Revenue	$ 59,391	$ 52,787
Percentage of revenue from contract with customer (as a percent)	11.00%	11.30%
Multi-Family and Residential
Disaggregation of Revenue
Revenue	$ 30,235	$ 33,052
Percentage of revenue from contract with customer (as a percent)	5.60%	7.10%
Other
Disaggregation of Revenue
Revenue	$ 22,931	$ 22,349
Percentage of revenue from contract with customer (as a percent)	4.30%	4.80%
New Construction
Disaggregation of Revenue
Revenue	$ 223,961	$ 185,282
Percentage of revenue from contract with customer (as a percent)	41.60%	39.90%
Existing Building Construction
Disaggregation of Revenue
Revenue	$ 182,296	$ 156,758
Percentage of revenue from contract with customer (as a percent)	33.80%	33.70%
Service Projects
Disaggregation of Revenue
Revenue	$ 50,384	$ 42,433
Percentage of revenue from contract with customer (as a percent)	9.40%	9.10%
Service Calls, Maintenance and Monitoring
Disaggregation of Revenue
Revenue	$ 81,833	$ 80,468
Percentage of revenue from contract with customer (as a percent)	15.20%	17.30%
HVAC and Plumbing
Disaggregation of Revenue
Revenue	$ 481,990	$ 424,017
Percentage of revenue from contract with customer (as a percent)	89.50%	91.20%
Building Automation Control Systems
Disaggregation of Revenue
Revenue	$ 29,014	$ 20,045
Percentage of revenue from contract with customer (as a percent)	5.40%	4.30%
Other
Disaggregation of Revenue
Revenue	$ 27,469	$ 20,879
Percentage of revenue from contract with customer (as a percent)	5.10%	4.50%
Minimum
Disaggregation of Revenue
Payments received term (in days)	30 days
Maximum
Disaggregation of Revenue
Payments received term (in days)	90 days