Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Segment Reporting Information [Line Items]
Revenues from unrelated entities	$ 2,521,672	$ 2,201,714	$ 7,263,387	$ 6,515,567
Total revenues	2,521,672	2,201,714	7,263,387	6,515,567
Operating income (loss)	137,421	135,855	387,783	119,212
Restructuring expenses	0	(536)	0	(605)
Impairment loss on goodwill, identifiable intangible assets, and other long-lived assets	0	0	0	(232,750)
Net periodic pension (cost) income	908	751	2,738	2,211
Interest expense, net	(1,286)	(1,484)	(3,965)	(6,082)
Income before income taxes	137,043	135,122	386,556	115,341
Total assets	5,296,678		5,296,678		$ 5,063,840
UNITED STATES
Segment Reporting Information [Line Items]
Revenues from unrelated entities	2,392,192	2,091,639	6,877,298	6,199,998
Total revenues	2,392,192	2,091,639	6,877,298	6,199,998
Operating income (loss)	154,934	155,518	439,105	406,147
Total assets	4,253,103		4,253,103		3,805,430
United States Electrical Construction And Facilities Services [Member] | UNITED STATES
Segment Reporting Information [Line Items]
Revenues from unrelated entities	527,935	472,084	1,473,590	1,344,275
United States Mechanical Construction And Facilities Services [Member] | UNITED STATES
Segment Reporting Information [Line Items]
Revenues from unrelated entities	999,584	891,509	2,862,195	2,516,062
United States Building Services [Member] | UNITED STATES
Segment Reporting Information [Line Items]
Revenues from unrelated entities	632,455	556,514	1,838,750	1,563,107
United States Industrial Services [Member] | UNITED STATES
Segment Reporting Information [Line Items]
Revenues from unrelated entities	232,218	171,532	702,763	776,554
United Kingdom Building Services [Member] [Member] | UNITED KINGDOM
Segment Reporting Information [Line Items]
Revenues from unrelated entities	129,480	110,075	386,089	315,569
Operating Segments [Member] | United States Electrical Construction And Facilities Services [Member] | UNITED STATES
Segment Reporting Information [Line Items]
Revenues from unrelated entities	528,591	473,892	1,476,034	1,347,729
Total revenues	528,720	474,438	1,476,410	1,348,646
Operating income (loss)	44,055	45,928	127,024	117,866
Total assets	842,143		842,143		672,226
Operating Segments [Member] | United States Mechanical Construction And Facilities Services [Member] | UNITED STATES
Segment Reporting Information [Line Items]
Revenues from unrelated entities	1,001,492	893,667	2,867,461	2,521,074
Total revenues	1,003,709	896,551	2,873,721	2,528,395
Operating income (loss)	82,302	80,048	226,511	192,156
Total assets	1,653,631		1,653,631		1,542,531
Operating Segments [Member] | United States Building Services [Member] | UNITED STATES
Segment Reporting Information [Line Items]
Total revenues	656,017	578,274	1,907,264	1,620,310
Operating income (loss)	31,589	38,449	91,233	86,325
Total assets	1,149,330		1,149,330		1,040,160
Operating Segments [Member] | United States Industrial Services [Member] | UNITED STATES
Segment Reporting Information [Line Items]
Total revenues	232,479	175,488	730,851	788,426
Operating income (loss)	(3,012)	(8,907)	(5,663)	9,800
Total assets	607,999		607,999		550,513
Operating Segments [Member] | United Kingdom Building Services [Member] [Member] | UNITED KINGDOM
Segment Reporting Information [Line Items]
Total revenues	129,480	110,075	386,089	315,569
Operating income (loss)	6,582	5,327	23,040	16,442
Total assets	246,711		246,711		227,894
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Total revenues	(28,733)	(33,112)	(110,948)	(85,779)
Intersegment Eliminations [Member] | United States Electrical Construction And Facilities Services [Member] | UNITED STATES
Segment Reporting Information [Line Items]
Revenues from unrelated entities	(656)	(1,808)	(2,444)	(3,454)
Intersegment Eliminations [Member] | United States Mechanical Construction And Facilities Services [Member] | UNITED STATES
Segment Reporting Information [Line Items]
Revenues from unrelated entities	(1,908)	(2,158)	(5,266)	(5,012)
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Operating income (loss)	(24,095)	$ (24,454)	(74,362)	$ (70,022)
Total assets	$ 796,864		$ 796,864		$ 1,030,516