Revenue from Contracts with Customers - Schedule of Disaggregation of Revenues (Details) - USD ($) $ in Thousands	3 Months Ended				9 Months Ended
	Sep. 30, 2019		Sep. 30, 2018		Sep. 30, 2019		Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Revenues	$ 2,287,741		$ 2,047,049		$ 6,770,671		$ 5,901,323
UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	2,190,113		1,946,483		6,452,962		5,588,360
United States Electrical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	554,637		485,974		1,652,109		1,420,268
United States Mechanical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	869,188		759,495		2,444,683		2,168,127
United States Building Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	532,122		473,701		1,567,899		1,389,486
United States Industrial Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	234,166		227,313		788,271		610,479
United Kingdom Building Services [Member] [Member] | UNITED KINGDOM
Disaggregation of Revenue [Line Items]
Revenues	97,628		100,566		317,709		312,963
Operating Segments [Member] | United States Electrical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	555,928		486,349		1,654,819		1,423,142
Operating Segments [Member] | United States Mechanical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	872,178		762,479		2,452,820		2,177,320
Intersegment Eliminations [Member] | United States Electrical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	(1,291)		(375)		(2,710)		(2,874)
Intersegment Eliminations [Member] | United States Mechanical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	(2,990)		(2,984)		(8,137)		(9,193)
Commercial Market Sector [Member] | Operating Segments [Member] | United States Electrical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	$ 260,871		$ 196,293		$ 816,602		$ 579,958
Percent of individual segment	47.00%		40.00%		49.00%		41.00%
Commercial Market Sector [Member] | Operating Segments [Member] | United States Mechanical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	$ 293,202		$ 287,350		$ 874,512		$ 780,629
Percent of individual segment	33.00%		38.00%		36.00%		36.00%
Institutional Market Sector [Member] | Operating Segments [Member] | United States Electrical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	$ 33,702		$ 27,903		$ 87,643		$ 86,971
Percent of individual segment	6.00%		6.00%		5.00%		6.00%
Institutional Market Sector [Member] | Operating Segments [Member] | United States Mechanical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	$ 94,886		$ 77,676		$ 232,843		$ 219,830
Percent of individual segment	11.00%		10.00%		9.00%		10.00%
Hospitality Market Sector [Member] | Operating Segments [Member] | United States Electrical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	$ 2,243		$ 9,860		$ 11,930		$ 22,665
Percent of individual segment	1.00%		2.00%		1.00%		2.00%
Hospitality Market Sector [Member] | Operating Segments [Member] | United States Mechanical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	$ 6,888		$ 26,170		$ 28,807		$ 73,888
Percent of individual segment	1.00%		3.00%		1.00%		3.00%
Manufacturing Market Sector [Member] | Operating Segments [Member] | United States Electrical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	$ 121,488		$ 100,563		$ 338,448		$ 280,375
Percent of individual segment	22.00%		20.00%		20.00%		20.00%
Manufacturing Market Sector [Member] | Operating Segments [Member] | United States Mechanical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	$ 150,199		$ 89,779		$ 386,853		$ 271,133
Percent of individual segment	17.00%		12.00%		16.00%		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	$ 24,701		$ 28,121		$ 63,725		$ 103,307
Percent of individual segment	4.00%		6.00%		4.00%		7.00%
Healthcare Market Sector [Member] | Operating Segments [Member] | United States Mechanical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	$ 84,418		$ 55,843		$ 220,445		$ 181,669
Percent of individual segment	10.00%		7.00%		9.00%		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	$ 52,744		$ 72,816		$ 162,538		$ 216,591
Percent of individual segment	9.00%		15.00%		10.00%		15.00%
Transportation Market Sector [Member] | Operating Segments [Member] | United States Mechanical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	$ 7,855		$ 3,072		$ 22,880		$ 13,002
Percent of individual segment	1.00%		1.00%		1.00%		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	$ 3,709		$ 4,194		$ 14,342		$ 14,947
Percent of individual segment	1.00%		1.00%		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	$ 42,637		$ 43,033		$ 131,385		$ 122,275
Percent of individual segment	5.00%		6.00%		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	$ 42,296	[1]	$ 38,452	[1]	$ 124,152	[2]	$ 93,177	[2]
Percent of individual segment	7.00%		8.00%		8.00%		6.00%
Short Duration Projects [Member] | Operating Segments [Member] | United States Mechanical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	$ 91,897	[1]	$ 72,388	[1]	$ 273,001	[3]	$ 223,200	[3]
Percent of individual segment	10.00%		9.00%		11.00%		10.00%
Service Work [Member] | United Kingdom Building Services [Member] [Member] | UNITED KINGDOM
Disaggregation of Revenue [Line Items]
Revenues	$ 49,098		$ 51,815		$ 160,561		$ 162,088
Percent of individual segment	50.00%		52.00%		51.00%		52.00%
Service Work [Member] | Operating Segments [Member] | United States Electrical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	$ 14,174		$ 8,147		$ 35,439		$ 25,151
Percent of individual segment	3.00%		2.00%		2.00%		2.00%
Service Work [Member] | Operating Segments [Member] | United States Mechanical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	$ 100,196		$ 107,168		$ 282,094		$ 291,694
Percent of individual segment	12.00%		14.00%		12.00%		13.00%
Commercial Site-Based Services [Member] | United States Building Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	$ 133,179		$ 120,917		$ 420,078		$ 386,721
Percent of individual segment	25.00%		26.00%		27.00%		28.00%
Government Site-Based Services [Member] | United States Building Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	$ 42,412		$ 52,332		$ 133,990		$ 162,888
Percent of individual segment	8.00%		11.00%		8.00%		12.00%
Mechanical Services [Member] | United States Building Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	$ 333,696		$ 276,722		$ 920,684		$ 764,313
Percent of individual segment	63.00%		58.00%		59.00%		55.00%
Energy Services [Member] | United States Building Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	$ 22,835		$ 23,730		$ 93,147		$ 75,564
Percent of individual segment	4.00%		5.00%		6.00%		5.00%
Field Services [Member] | United States Industrial Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	$ 194,569		$ 181,873		$ 664,188		$ 486,362
Percent of individual segment	83.00%		80.00%		84.00%		80.00%
Shop Services [Member] | United States Industrial Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	$ 39,597		$ 45,440		$ 124,083		$ 124,117
Percent of individual segment	17.00%		20.00%		16.00%		20.00%
Projects & Extras [Member] | United Kingdom Building Services [Member] [Member] | UNITED KINGDOM
Disaggregation of Revenue [Line Items]
Revenues	$ 48,530		$ 48,751		$ 157,148		$ 150,875
Percent of individual segment	50.00%		48.00%		49.00%		48.00%

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

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

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