Revenue from Contracts with Customers - Schedule of Disaggregation of Revenues (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Revenues		$ 2,229,308	$ 2,047,049	$ 1,953,886	$ 1,900,388	$ 2,012,639	$ 1,886,691	$ 1,895,937	$ 1,891,732	$ 8,130,631	$ 7,686,999	$ 7,551,524
UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues										7,715,760	7,346,254	7,225,268
United States Electrical Construction And Facilities Services | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues										1,954,323	1,829,567	1,704,403
United States Mechanical Construction And Facilities Services | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues										3,020,307	2,963,815	2,643,321
United States Building Services | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues										1,875,485	1,753,703	1,810,229
United States Industrial Services | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues										865,645	799,169	1,067,315
United Kingdom Building Services | UNITED KINGDOM
Disaggregation of Revenue [Line Items]
Revenues										414,871	$ 340,745	$ 326,256
Operating Segments [Member] | United States Electrical Construction And Facilities Services | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues										1,957,819
Operating Segments [Member] | United States Mechanical Construction And Facilities Services | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues										3,038,665
Intersegment Eliminations [Member] | United States Electrical Construction And Facilities Services | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues										(3,496)
Intersegment Eliminations [Member] | United States Mechanical Construction And Facilities Services | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues										(18,358)
Commercial Market Sector [Member] | Operating Segments [Member] | United States Electrical Construction And Facilities Services | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues										$ 839,045
Percent of individual segment										43.00%
Commercial Market Sector [Member] | Operating Segments [Member] | United States Mechanical Construction And Facilities Services | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues										$ 1,063,571
Percent of individual segment										35.00%
Institutional Market Sector [Member] | Operating Segments [Member] | United States Electrical Construction And Facilities Services | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues										$ 110,046
Percent of individual segment										6.00%
Institutional Market Sector [Member] | Operating Segments [Member] | United States Mechanical Construction And Facilities Services | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues										$ 292,255
Percent of individual segment										9.00%
Hospitality Market Sector [Member] | Operating Segments [Member] | United States Electrical Construction And Facilities Services | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues										$ 32,338
Percent of individual segment										2.00%
Hospitality Market Sector [Member] | Operating Segments [Member] | United States Mechanical Construction And Facilities Services | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues										$ 93,827
Percent of individual segment										3.00%
Manufacturing Market Sector [Member] | Operating Segments [Member] | United States Electrical Construction And Facilities Services | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues										$ 388,157
Percent of individual segment										20.00%
Manufacturing Market Sector [Member] | Operating Segments [Member] | United States Mechanical Construction And Facilities Services | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues										$ 430,286
Percent of individual segment										14.00%
Healthcare Market Sector [Member] | Operating Segments [Member] | United States Electrical Construction And Facilities Services | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues										$ 126,218
Percent of individual segment										6.00%
Healthcare Market Sector [Member] | Operating Segments [Member] | United States Mechanical Construction And Facilities Services | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues										$ 241,113
Percent of individual segment										8.00%
Transportation Market Sector [Member] | Operating Segments [Member] | United States Electrical Construction And Facilities Services | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues										$ 284,464
Percent of individual segment										14.00%
Transportation Market Sector [Member] | Operating Segments [Member] | United States Mechanical Construction And Facilities Services | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues										$ 19,415
Percent of individual segment										1.00%
Water and Wastewater Market Sector [Member] | Operating Segments [Member] | United States Electrical Construction And Facilities Services | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues										$ 23,337
Percent of individual segment										1.00%
Water and Wastewater Market Sector [Member] | Operating Segments [Member] | United States Mechanical Construction And Facilities Services | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues										$ 176,574
Percent of individual segment										6.00%
Short Duration Projects [Member] | Operating Segments [Member] | United States Electrical Construction And Facilities Services | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues										$ 120,109
Percent of individual segment										6.00%
Short Duration Projects [Member] | Operating Segments [Member] | United States Mechanical Construction And Facilities Services | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues	[1]									$ 335,953
Percent of individual segment										11.00%
Service Work [Member] | Operating Segments [Member] | United States Electrical Construction And Facilities Services | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues										$ 34,105
Percent of individual segment										2.00%
Service Work [Member] | Operating Segments [Member] | United States Mechanical Construction And Facilities Services | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues										$ 385,671
Percent of individual segment										13.00%
Service Work [Member] | Operating Segments [Member] | United Kingdom Building Services | UNITED KINGDOM
Disaggregation of Revenue [Line Items]
Revenues										$ 216,880
Percent of individual segment										52.00%
Commercial Site-Based Services [Member] | Operating Segments [Member] | United States Building Services | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues										$ 519,641
Percent of individual segment										28.00%
Government Site-Based Services [Member] | Operating Segments [Member] | United States Building Services | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues										$ 213,677
Percent of individual segment										11.00%
Mechanical Services [Member] | Operating Segments [Member] | United States Building Services | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues										$ 1,032,082
Percent of individual segment										55.00%
Energy Services [Member] | Operating Segments [Member] | United States Building Services | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues										$ 110,085
Percent of individual segment										6.00%
Field Services [Member] | Operating Segments [Member] | United States Industrial Services | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues										$ 694,994
Percent of individual segment										80.00%
Shop Services [Member] | Operating Segments [Member] | United States Industrial Services | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenues										$ 170,651
Percent of individual segment										20.00%
Projects & Extras [Member] | Operating Segments [Member] | United Kingdom Building Services | UNITED KINGDOM
Disaggregation of Revenue [Line Items]
Revenues										$ 197,991
Percent of individual segment										48.00%

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