Segment Information (Information About Industry Segments And Geographic Areas) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Revenues from unrelated entities	$ 1,777,826	$ 1,699,128	$ 1,652,585	$ 1,589,187	$ 1,714,796	$ 1,566,711	$ 1,552,919	$ 1,590,539	$ 6,718,726	$ 6,424,965	$ 6,333,527
Total revenues									6,718,726	6,424,965	6,333,527
Operating income (loss)									287,082	289,878	240,350
Restructuring expenses									(824)	(1,168)	(647)
Impairment loss on identifiable intangible assets	0	0	0	0	1,471	0	0	0	0	1,471	0
Gain on sale of building	0	$ 0	$ 0	$ 0	0	$ 11,749	0	$ 0	0	11,749	0
Interest expense									(8,932)	(9,075)	(8,769)
Interest income									673	842	1,128
Income from continuing operations before income taxes									278,823	281,645	232,709
Capital expenditures									35,460	38,035	35,497
Depreciation and amortization of Property, plant and equipment									36,294	36,524	36,310
Costs and estimated earnings in excess of billings on uncompleted contracts	117,734				103,201				117,734	103,201	90,727
Billings in excess of costs and estimated earnings on uncompleted contracts	429,235				368,555				429,235	368,555	381,295
Long-lived assets	1,438,022				1,458,340				1,438,022	1,458,340	1,499,736
Total assets	3,546,470				3,388,967				3,546,470	3,388,967	3,465,915
Settlement of contract claim	12,100								12,100	0
Income from reduction in estimate of certain accrued contract costs							$ 4,800
Insurance recoveries											2,600
UNITED STATES
Segment Reporting Information [Line Items]
Revenues from unrelated entities									6,341,249	6,074,521	5,989,975
Total revenues									6,341,249	6,074,521	5,989,975
Operating income (loss)									347,914	334,335	297,867
Capital expenditures									29,714	35,030	32,890
Depreciation and amortization of Property, plant and equipment									31,763	32,336	30,989
Costs and estimated earnings in excess of billings on uncompleted contracts	109,653				94,743				109,653	94,743	82,754
Billings in excess of costs and estimated earnings on uncompleted contracts	423,966				353,980				423,966	353,980	356,064
Long-lived assets	1,425,552				1,449,418				1,425,552	1,449,418	1,489,009
Total assets	2,871,882				2,816,306				2,871,882	2,816,306	2,822,699
UNITED STATES | United States electrical construction and facilities services segment
Segment Reporting Information [Line Items]
Revenues from unrelated entities									1,367,142	1,311,988	1,345,750
UNITED STATES | United States Mechanical Construction And Facilities Services
Segment Reporting Information [Line Items]
Revenues from unrelated entities									2,312,763	2,201,212	2,329,834
UNITED STATES | United States Building Services
Segment Reporting Information [Line Items]
Revenues from unrelated entities									1,739,259	1,721,341	1,794,978
UNITED STATES | United States Industrial Services
Segment Reporting Information [Line Items]
Revenues from unrelated entities									922,085	839,980	519,413
UNITED KINGDOM | United Kingdom Building Services
Segment Reporting Information [Line Items]
Revenues from unrelated entities									377,477	350,444	343,552
Operating Segments [Member] | UNITED STATES | United States electrical construction and facilities services segment
Segment Reporting Information [Line Items]
Total revenues									1,378,620	1,326,547	1,371,979
Operating income (loss)									82,225	90,873	98,114
Restructuring expenses										(638)
Capital expenditures									6,063	6,671	6,164
Depreciation and amortization of Property, plant and equipment									4,676	4,237	3,640
Costs and estimated earnings in excess of billings on uncompleted contracts	39,116				32,464				39,116	32,464	28,988
Billings in excess of costs and estimated earnings on uncompleted contracts	139,857				114,422				139,857	114,422	118,458
Long-lived assets	20,139				18,792				20,139	18,792	16,512
Total assets	372,525				332,150				372,525	332,150	329,742
Operating Segments [Member] | UNITED STATES | United States Mechanical Construction And Facilities Services
Segment Reporting Information [Line Items]
Total revenues									2,326,683	2,219,886	2,387,072
Operating income (loss)									138,688	114,418	93,765
Restructuring expenses									(6)	(230)
Capital expenditures									5,345	8,631	8,866
Depreciation and amortization of Property, plant and equipment									7,624	7,600	7,280
Costs and estimated earnings in excess of billings on uncompleted contracts	46,220				43,443				46,220	43,443	38,804
Billings in excess of costs and estimated earnings on uncompleted contracts	238,463				199,983				238,463	199,983	205,974
Long-lived assets	296,633				288,161				296,633	288,161	293,790
Total assets	894,366				793,056				894,366	793,056	795,256
Operating Segments [Member] | UNITED STATES | United States Building Services
Segment Reporting Information [Line Items]
Total revenues									1,794,086	1,762,697	1,839,129
Operating income (loss)									70,532	65,885	67,225
Restructuring expenses									(924)	0
Capital expenditures									7,233	10,589	7,579
Depreciation and amortization of Property, plant and equipment									9,834	10,660	11,288
Costs and estimated earnings in excess of billings on uncompleted contracts	20,959				18,555				20,959	18,555	14,957
Billings in excess of costs and estimated earnings on uncompleted contracts	44,476				38,059				44,476	38,059	30,827
Long-lived assets	378,367				392,364				378,367	392,364	406,498
Total assets	721,653				737,082				721,653	737,082	756,785
Operating Segments [Member] | UNITED STATES | United States Industrial Services
Segment Reporting Information [Line Items]
Total revenues									923,648	842,040	522,417
Operating income (loss)									56,469	63,159	38,763
Capital expenditures									11,073	9,139	10,281
Depreciation and amortization of Property, plant and equipment									9,629	9,839	8,781
Costs and estimated earnings in excess of billings on uncompleted contracts	3,358				281				3,358	281	5
Billings in excess of costs and estimated earnings on uncompleted contracts	1,170				1,516				1,170	1,516	805
Long-lived assets	730,413				750,101				730,413	750,101	772,209
Total assets	883,338				954,018				883,338	954,018	940,916
Operating Segments [Member] | UNITED KINGDOM | United Kingdom Building Services
Segment Reporting Information [Line Items]
Total revenues									377,477	350,444	343,552
Operating income (loss)									11,634	15,011	13,021
Capital expenditures									5,298	1,935	1,536
Depreciation and amortization of Property, plant and equipment									3,603	3,305	4,477
Costs and estimated earnings in excess of billings on uncompleted contracts	8,081				8,458				8,081	8,458	7,973
Billings in excess of costs and estimated earnings on uncompleted contracts	5,269				14,575				5,269	14,575	25,231
Long-lived assets	10,927				6,899				10,927	6,899	8,831
Total assets	133,782				130,340				133,782	130,340	160,828
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Operating income (loss)									(71,642)	(68,578)	(69,891)
Restructuring expenses									0	(300)
Capital expenditures									448	1,070	1,071
Depreciation and amortization of Property, plant and equipment									928	883	844
Long-lived assets	1,543				2,023				1,543	2,023	1,896
Total assets	$ 540,806				$ 442,321				540,806	442,321	482,388
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Total revenues									$ (81,788)	$ (76,649)	$ (130,622)