Segment Information (Information About Industry Segments And Geographic Areas) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Segment Reporting Information [Line Items]
Revenues from unrelated entities	$ 2,287,741	$ 2,047,049	$ 6,770,671	$ 5,901,323
Total revenues	2,287,741	2,047,049	6,770,671	5,901,323
Operating income (loss)	115,749	111,776	338,018	289,436
Restructuring expenses	(119)	(229)	(567)	(693)
Impairment loss on identifiable intangible assets	0	0	0	(907)
Net periodic pension (cost) income	381	615	1,187	2,069
Interest expense, net	(2,678)	(2,736)	(8,732)	(8,011)
Income from continuing operations before income taxes	113,452	109,655	330,473	283,494
Total assets	4,532,721		4,532,721		$ 4,088,807
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Total revenues	(40,806)	(28,887)	(99,491)	(82,226)
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Operating income (loss)	(24,341)	(23,113)	(74,062)	(65,508)
Total assets	463,874		463,874		448,644
UNITED STATES
Segment Reporting Information [Line Items]
Revenues from unrelated entities	2,190,113	1,946,483	6,452,962	5,588,360
Total revenues	2,190,113	1,946,483	6,452,962	5,588,360
Operating income (loss)	135,455	130,661	398,276	343,716
Total assets	3,905,778		3,905,778		3,493,784
UNITED STATES | United States Electrical Construction And Facilities Services [Member]
Segment Reporting Information [Line Items]
Revenues from unrelated entities	554,637	485,974	1,652,109	1,420,268
UNITED STATES | United States Mechanical Construction And Facilities Services [Member]
Segment Reporting Information [Line Items]
Revenues from unrelated entities	869,188	759,495	2,444,683	2,168,127
UNITED STATES | United States Building Services [Member]
Segment Reporting Information [Line Items]
Revenues from unrelated entities	532,122	473,701	1,567,899	1,389,486
UNITED STATES | United States Industrial Services [Member]
Segment Reporting Information [Line Items]
Revenues from unrelated entities	234,166	227,313	788,271	610,479
UNITED STATES | Operating Segments [Member] | United States Electrical Construction And Facilities Services [Member]
Segment Reporting Information [Line Items]
Revenues from unrelated entities	555,928	486,349	1,654,819	1,423,142
Total revenues	556,088	486,642	1,655,147	1,425,190
Operating income (loss)	33,630	34,473	120,380	106,309
Restructuring expenses			0	0
Total assets	820,229		820,229		702,112
UNITED STATES | Operating Segments [Member] | United States Mechanical Construction And Facilities Services [Member]
Segment Reporting Information [Line Items]
Revenues from unrelated entities	872,178	762,479	2,452,820	2,177,320
Total revenues	877,571	770,617	2,465,287	2,195,951
Operating income (loss)	61,213	59,236	156,152	156,273
Total assets	1,228,920		1,228,920		1,080,829
UNITED STATES | Operating Segments [Member] | United States Building Services [Member]
Segment Reporting Information [Line Items]
Total revenues	550,002	490,624	1,622,509	1,437,498
Operating income (loss)	35,051	29,315	90,535	68,822
Restructuring expenses			(567)	(693)
Total assets	984,514		984,514		846,221
UNITED STATES | Operating Segments [Member] | United States Industrial Services [Member]
Segment Reporting Information [Line Items]
Total revenues	247,258	227,487	809,510	611,947
Operating income (loss)	5,561	7,637	31,209	12,312
Total assets	872,115		872,115		864,622
UNITED STATES | Operating Segments [Member] | Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Restructuring expenses			0	0
UNITED STATES | Intersegment Eliminations [Member] | United States Electrical Construction And Facilities Services [Member]
Segment Reporting Information [Line Items]
Revenues from unrelated entities	(1,291)	(375)	(2,710)	(2,874)
UNITED STATES | Intersegment Eliminations [Member] | United States Mechanical Construction And Facilities Services [Member]
Segment Reporting Information [Line Items]
Revenues from unrelated entities	(2,990)	(2,984)	(8,137)	(9,193)
UNITED KINGDOM | United Kingdom Building Services [Member] [Member]
Segment Reporting Information [Line Items]
Revenues from unrelated entities	97,628	100,566	317,709	312,963
UNITED KINGDOM | Operating Segments [Member] | United Kingdom Building Services [Member] [Member]
Segment Reporting Information [Line Items]
Total revenues	97,628	100,566	317,709	312,963
Operating income (loss)	4,754	$ 4,457	14,371	$ 12,828
Total assets	$ 163,069		$ 163,069		$ 146,379