Segment Information (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2020 USD ($) segment	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Segment Information
Number of reportable segments | segment									2
Revenue	$ 698,961	$ 714,099	$ 743,468	$ 700,131	$ 719,584	$ 706,918	$ 650,302	$ 538,473	$ 2,856,659	$ 2,615,277	$ 2,182,879
Gross Profit	136,999	$ 147,196	$ 145,695	$ 117,093	132,560	$ 142,702	$ 120,016	$ 106,665	546,983	501,943	446,279
Capital Expenditures									24,131	31,750	27,268
Total Assets	1,757,355				1,505,012				1,757,355	1,505,012
Operating | Mechanical Services
Segment Information
Revenue									2,413,016	2,251,560	2,176,223
Gross Profit									509,740	465,144	444,960
Capital Expenditures									22,550	27,933	25,945
Total Assets	1,215,985				1,056,609				1,215,985	1,056,609
Operating | Electrical Services
Segment Information
Revenue									443,643	363,717	6,656
Gross Profit									37,243	36,799	1,319
Capital Expenditures									955	1,504	57
Total Assets	449,588				372,254				449,588	372,254
Corporate & Eliminations
Segment Information
Capital Expenditures									626	2,313	$ 1,266
Total Assets	$ 91,782				$ 76,149				$ 91,782	$ 76,149