Revenue from Contracts with Customers - Schedule of Disaggregation of Revenues (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019		Jun. 30, 2018		Jun. 30, 2019		Jun. 30, 2018
Disaggregation of Revenue [Line Items]
Revenues	$ 2,324,202		$ 1,953,886		$ 4,482,930		$ 3,854,274
UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	2,211,646		1,848,379		4,262,849		3,641,877
United States Electrical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	569,402		479,542		1,097,472		934,294
United States Mechanical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	823,086		723,882		1,575,495		1,408,632
United States Building Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	523,698		461,033		1,035,777		915,785
United States Industrial Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	295,460		183,922		554,105		383,166
United Kingdom Building Services [Member] [Member] | UNITED KINGDOM
Disaggregation of Revenue [Line Items]
Revenues	112,556		105,507		220,081		212,397
Operating Segments [Member] | United States Electrical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	570,021		481,011		1,098,891		936,793
Operating Segments [Member] | United States Mechanical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	825,365		726,866		1,580,642		1,414,841
Intersegment Eliminations [Member] | United States Electrical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	(619)		(1,469)		(1,419)		(2,499)
Intersegment Eliminations [Member] | United States Mechanical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	(2,279)		(2,984)		(5,147)		(6,209)
Commercial Market Sector [Member] | Operating Segments [Member] | United States Electrical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	$ 286,290		$ 199,283		$ 555,731		$ 383,665
Percent of individual segment	50.00%		41.00%		51.00%		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	$ 299,448		$ 262,480		$ 581,310		$ 493,279
Percent of individual segment	36.00%		36.00%		37.00%		35.00%
Institutional Market Sector [Member] | Operating Segments [Member] | United States Electrical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	$ 31,596		$ 31,060		$ 53,941		$ 59,068
Percent of individual segment	5.00%		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	$ 76,674		$ 77,499		$ 137,957		$ 142,154
Percent of individual segment	9.00%		11.00%		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	$ 2,846		$ 7,289		$ 9,687		$ 12,805
Percent of individual segment	1.00%		2.00%		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	$ 8,271		$ 21,191		$ 21,919		$ 47,718
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	$ 118,157		$ 94,018		$ 216,960		$ 179,812
Percent of individual segment	21.00%		20.00%		20.00%		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	$ 137,891		$ 89,140		$ 236,654		$ 181,354
Percent of individual segment	17.00%		12.00%		15.00%		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	$ 21,409		$ 36,679		$ 39,024		$ 75,186
Percent of individual segment	4.00%		8.00%		3.00%		8.00%
Healthcare Market Sector [Member] | Operating Segments [Member] | United States Mechanical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	$ 74,653		$ 59,950		$ 136,027		$ 125,826
Percent of individual segment	9.00%		8.00%		9.00%		9.00%
Transportation Market Sector [Member] | Operating Segments [Member] | United States Electrical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	$ 51,655		$ 72,511		$ 109,794		$ 143,775
Percent of individual segment	9.00%		15.00%		10.00%		16.00%
Transportation Market Sector [Member] | Operating Segments [Member] | United States Mechanical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	$ 9,291		$ 4,724		$ 15,025		$ 9,930
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	$ 4,618		$ 6,140		$ 10,633		$ 10,753
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	$ 45,537		$ 42,491		$ 88,748		$ 79,242
Percent of individual segment	6.00%		6.00%		6.00%		5.00%
Short Duration Projects [Member] | Operating Segments [Member] | United States Electrical Construction And Facilities Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	$ 41,447	[1]	$ 25,195	[1]	$ 81,856	[2]	$ 54,725	[2]
Percent of individual segment	7.00%		5.00%		7.00%		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	$ 81,168	[1]	$ 71,090	[1]	$ 181,104	[3]	$ 150,812	[3]
Percent of individual segment	10.00%		10.00%		11.00%		11.00%
Service Work [Member] | United Kingdom Building Services [Member] [Member] | UNITED KINGDOM
Disaggregation of Revenue [Line Items]
Revenues	$ 56,829		$ 54,998		$ 111,463		$ 110,273
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	$ 12,003		$ 8,836		$ 21,265		$ 17,004
Percent of individual segment	2.00%		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	$ 92,432		$ 98,301		$ 181,898		$ 184,526
Percent of individual segment	11.00%		13.00%		11.00%		13.00%
Commercial Site-Based Services [Member] | United States Building Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	$ 130,921		$ 119,043		$ 286,899		$ 265,804
Percent of individual segment	25.00%		26.00%		28.00%		29.00%
Government Site-Based Services [Member] | United States Building Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	$ 42,787		$ 55,147		$ 91,578		$ 110,556
Percent of individual segment	8.00%		12.00%		9.00%		12.00%
Mechanical Services [Member] | United States Building Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	$ 312,822		$ 260,249		$ 586,988		$ 487,591
Percent of individual segment	60.00%		56.00%		56.00%		53.00%
Energy Services [Member] | United States Building Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	$ 37,168		$ 26,594		$ 70,312		$ 51,834
Percent of individual segment	7.00%		6.00%		7.00%		6.00%
Field Services [Member] | United States Industrial Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	$ 252,849		$ 142,302		$ 469,619		$ 304,489
Percent of individual segment	86.00%		77.00%		85.00%		79.00%
Shop Services [Member] | United States Industrial Services [Member] | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	$ 42,611		$ 41,620		$ 84,486		$ 78,677
Percent of individual segment	14.00%		23.00%		15.00%		21.00%
Projects & Extras [Member] | United Kingdom Building Services [Member] [Member] | UNITED KINGDOM
Disaggregation of Revenue [Line Items]
Revenues	$ 55,727		$ 50,509		$ 108,618		$ 102,124
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.