Segment Information (Information About Industry Segments And Geographic Areas) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Segment Reporting Information [Line Items]
Revenues from unrelated entities	$ 1,886,691	$ 1,923,174	$ 5,674,360	$ 5,601,560
Total revenues	1,886,691	1,923,174	5,674,360	5,601,560
Operating income (loss)	106,459	86,064	282,054	233,968
Restructuring expenses	(46)	(539)	(954)	(1,271)
Interest expense	(3,324)	(3,479)	(9,464)	(8,973)
Interest income	277	161	607	518
Income from continuing operations before income taxes	103,412	82,746	273,197	225,513
Total assets	3,966,321		3,966,321		$ 3,852,438
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Total revenues	(25,204)	(26,482)	(75,992)	(74,938)
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Operating income (loss)	(22,632)	(23,516)	(63,755)	(67,431)
Total assets	577,919		577,919		557,275
UNITED STATES
Segment Reporting Information [Line Items]
Revenues from unrelated entities	1,800,789	1,850,138	5,430,282	5,350,304
Total revenues	1,800,789	1,850,138	5,430,282	5,350,304
Operating income (loss)	125,204	107,528	337,654	293,510
Total assets	3,266,743		3,266,743		3,190,082
UNITED STATES | United States Electrical Construction And Facilities Services [Member]
Segment Reporting Information [Line Items]
Revenues from unrelated entities	457,919	458,553	1,350,157	1,227,474
UNITED STATES | United States Mechanical Construction And Facilities Services [Member]
Segment Reporting Information [Line Items]
Revenues from unrelated entities	760,084	691,818	2,173,030	1,925,793
UNITED STATES | United States Building Services [Member]
Segment Reporting Information [Line Items]
Revenues from unrelated entities	437,107	460,715	1,315,401	1,366,973
UNITED STATES | United States Industrial Services [Member]
Segment Reporting Information [Line Items]
Revenues from unrelated entities	145,679	239,052	591,694	830,064
UNITED STATES | Operating Segments [Member] | United States Electrical Construction And Facilities Services [Member]
Segment Reporting Information [Line Items]
Total revenues	460,066	465,476	1,355,206	1,249,681
Operating income (loss)	46,583	30,927	109,735	70,645
Total assets	595,680		595,680		631,581
UNITED STATES | Operating Segments [Member] | United States Mechanical Construction And Facilities Services [Member]
Segment Reporting Information [Line Items]
Total revenues	767,398	696,549	2,197,231	1,937,089
Operating income (loss)	57,484	38,890	150,971	100,607
Restructuring expenses			(181)	(401)
Total assets	1,055,857		1,055,857		954,633
UNITED STATES | Operating Segments [Member] | United States Building Services [Member]
Segment Reporting Information [Line Items]
Total revenues	451,396	475,106	1,360,189	1,407,361
Operating income (loss)	25,981	23,125	60,375	55,658
Restructuring expenses			(773)	(870)
Total assets	774,449		774,449		753,434
UNITED STATES | Operating Segments [Member] | United States Industrial Services [Member]
Segment Reporting Information [Line Items]
Total revenues	147,133	239,489	593,648	831,111
Operating income (loss)	(4,844)	14,586	16,573	66,600
Total assets	840,757		840,757		850,434
UNITED KINGDOM | United Kingdom Building Services [Member] [Member]
Segment Reporting Information [Line Items]
Revenues from unrelated entities	85,902	73,036	244,078	251,256
UNITED KINGDOM | Operating Segments [Member] | United Kingdom Building Services [Member] [Member]
Segment Reporting Information [Line Items]
Total revenues	85,902	73,036	244,078	251,256
Operating income (loss)	3,933	$ 2,591	9,109	$ 9,160
Total assets	$ 121,659		$ 121,659		$ 105,081