Revenue from Contracts with Customers - Schedule of Disaggregation of Revenues (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Revenues		$ 2,299,832	$ 2,158,728
UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues		2,187,455	2,051,203
United States Electrical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues		525,229	528,070
United States Mechanical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues		834,112	752,409
United States Building Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues		518,083	512,079
United States Industrial Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues		310,031	258,645
United Kingdom Building Services [Member] [Member] | UNITED KINGDOM
Disaggregation of Revenue [Line Items]
Revenues		112,377	107,525
Operating Segments [Member] | United States Electrical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues		525,985	528,870
Operating Segments [Member] | United States Mechanical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues		835,996	755,277
Intersegment Eliminations [Member] | United States Electrical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues		(756)	(800)
Intersegment Eliminations [Member] | United States Mechanical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues		(1,884)	(2,868)
Commercial Market Sector [Member] | Operating Segments [Member] | United States Electrical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues		$ 242,841	$ 269,441
Percent of individual segment		46.00%	51.00%
Commercial Market Sector [Member] | Operating Segments [Member] | United States Mechanical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues		$ 304,690	$ 281,862
Percent of individual segment		36.00%	37.00%
Institutional Market Sector [Member] | Operating Segments [Member] | United States Electrical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues		$ 41,518	$ 22,345
Percent of individual segment		8.00%	4.00%
Institutional Market Sector [Member] | Operating Segments [Member] | United States Mechanical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues		$ 76,997	$ 61,283
Percent of individual segment		9.00%	8.00%
Hospitality Market Sector [Member] | Operating Segments [Member] | United States Electrical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues		$ 5,082	$ 6,841
Percent of individual segment		1.00%	1.00%
Hospitality Market Sector [Member] | Operating Segments [Member] | United States Mechanical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues		$ 7,714	$ 13,648
Percent of individual segment		1.00%	2.00%
Manufacturing Market Sector [Member] | Operating Segments [Member] | United States Electrical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues		$ 120,372	$ 98,803
Percent of individual segment		23.00%	19.00%
Manufacturing Market Sector [Member] | Operating Segments [Member] | United States Mechanical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues		$ 115,582	$ 98,763
Percent of individual segment		14.00%	13.00%
Healthcare Market Sector [Member] | Operating Segments [Member] | United States Electrical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues		$ 20,646	$ 17,615
Percent of individual segment		4.00%	3.00%
Healthcare Market Sector [Member] | Operating Segments [Member] | United States Mechanical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues		$ 88,059	$ 61,374
Percent of individual segment		10.00%	8.00%
Transportation Market Sector [Member] | Operating Segments [Member] | United States Electrical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues		$ 44,180	$ 58,139
Percent of individual segment		8.00%	11.00%
Transportation Market Sector [Member] | Operating Segments [Member] | United States Mechanical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues		$ 14,346	$ 5,734
Percent of individual segment		2.00%	1.00%
Water and Wastewater Market Sector [Member] | Operating Segments [Member] | United States Electrical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues		$ 2,329	$ 6,015
Percent of individual segment		1.00%	1.00%
Water and Wastewater Market Sector [Member] | Operating Segments [Member] | United States Mechanical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues		$ 40,513	$ 43,211
Percent of individual segment		5.00%	6.00%
Short Duration Projects [Member] | Operating Segments [Member] | United States Electrical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	[1]	$ 32,721	$ 40,409
Percent of individual segment		6.00%	8.00%
Short Duration Projects [Member] | Operating Segments [Member] | United States Mechanical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	[1]	$ 97,202	$ 99,936
Percent of individual segment		12.00%	13.00%
Service Work [Member] | United Kingdom Building Services [Member] [Member] | UNITED KINGDOM
Disaggregation of Revenue [Line Items]
Revenues		$ 55,106	$ 54,634
Percent of individual segment		49.00%	51.00%
Service Work [Member] | Operating Segments [Member] | United States Electrical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues		$ 16,296	$ 9,262
Percent of individual segment		3.00%	2.00%
Service Work [Member] | Operating Segments [Member] | United States Mechanical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues		$ 90,893	$ 89,466
Percent of individual segment		11.00%	12.00%
Commercial Site-Based Services [Member] | United States Building Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues		$ 143,438	$ 155,978
Percent of individual segment		28.00%	30.00%
Government Site-Based Services [Member] | United States Building Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues		$ 42,917	$ 48,791
Percent of individual segment		8.00%	10.00%
Mechanical Services [Member] | United States Building Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues		$ 303,391	$ 274,166
Percent of individual segment		59.00%	54.00%
Energy Services [Member] | United States Building Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues		$ 28,337	$ 33,144
Percent of individual segment		5.00%	6.00%
Field Services [Member] | United States Industrial Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues		$ 269,756	$ 216,770
Percent of individual segment		87.00%	84.00%
Shop Services [Member] | United States Industrial Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues		$ 40,275	$ 41,875
Percent of individual segment		13.00%	16.00%
Projects & Extras [Member] | United Kingdom Building Services [Member] [Member] | UNITED KINGDOM
Disaggregation of Revenue [Line Items]
Revenues		$ 57,271	$ 52,891
Percent of individual segment		51.00%	49.00%

[1] Represents those projects which generally are completed within three months or less.