Segment Information (Information About Industry Segments And Geographic Areas) (Details) - USD ($)	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
Segment Reporting Information [Line Items]
Revenues from unrelated entities	$ 2,229,308,000	$ 2,047,049,000	$ 1,953,886,000	$ 1,900,388,000	$ 2,012,639,000	$ 1,886,691,000	$ 1,895,937,000	$ 1,891,732,000	$ 8,130,631,000	$ 7,686,999,000	$ 7,551,524,000
Total revenues									8,130,631,000	7,686,999,000	7,551,524,000
Operating income (loss)									403,083,000	328,902,000	306,929,000
Net periodic pension (cost) income									2,743,000	1,652,000	1,529,000
Restructuring expenses									(2,306,000)	(1,577,000)	(1,438,000)
Impairment loss on goodwill and identifiable intangible assets									(907,000)	(57,819,000)	(2,428,000)
Interest expense									(13,544,000)	(12,770,000)	(12,627,000)
Interest income									2,746,000	965,000	663,000
Income from continuing operations before income taxes									395,028,000	318,749,000	296,494,000
Capital expenditures									43,479,000	34,684,000	39,648,000
Depreciation and amortization of Property, plant and equipment									38,472,000	39,915,000	38,881,000
Contract assets	158,243,000				122,621,000				158,243,000	122,621,000	130,697,000
Contract liabilities	552,290,000				524,156,000				552,290,000	524,156,000	489,242,000
Long-lived assets	1,613,524,000				1,587,085,000				1,613,524,000	1,587,085,000	1,594,977,000
Total assets	4,088,807,000				3,965,904,000				4,088,807,000	3,965,904,000	3,852,438,000
Loss on contracts										0
United States electrical construction and facilities services segment | Transportation Sector [Member]
Segment Reporting Information [Line Items]
Loss on contracts									10,000,000		19,400,000
United States Mechanical Construction And Facilities Services
Segment Reporting Information [Line Items]
Settlement of contract claim										18,100,000
United States Mechanical Construction And Facilities Services | Manufacturing Sector [Member]
Segment Reporting Information [Line Items]
Loss on contracts											18,300,000
United States Mechanical Construction And Facilities Services | Institutional Sector [Member]
Segment Reporting Information [Line Items]
Loss on contracts											9,600,000
UNITED STATES
Segment Reporting Information [Line Items]
Revenues from unrelated entities									7,715,760,000	7,346,254,000	7,225,268,000
Total revenues									7,715,760,000	7,346,254,000	7,225,268,000
Operating income (loss)									480,781,000	463,201,000	389,449,000
Capital expenditures									42,297,000	31,903,000	34,111,000
Depreciation and amortization of Property, plant and equipment									34,291,000	35,689,000	34,497,000
Contract assets	135,135,000				115,105,000				135,135,000	115,105,000	123,552,000
Contract liabilities	536,385,000				520,249,000				536,385,000	520,249,000	487,974,000
Long-lived assets	1,603,188,000				1,573,862,000				1,603,188,000	1,573,862,000	1,581,797,000
Total assets	3,493,784,000				3,251,695,000				3,493,784,000	3,251,695,000	3,190,082,000
UNITED STATES | United States electrical construction and facilities services segment
Segment Reporting Information [Line Items]
Revenues from unrelated entities									1,954,323,000	1,829,567,000	1,704,403,000
UNITED STATES | United States Mechanical Construction And Facilities Services
Segment Reporting Information [Line Items]
Revenues from unrelated entities									3,020,307,000	2,963,815,000	2,643,321,000
UNITED STATES | United States Building Services
Segment Reporting Information [Line Items]
Revenues from unrelated entities									1,875,485,000	1,753,703,000	1,810,229,000
UNITED STATES | United States Industrial Services
Segment Reporting Information [Line Items]
Revenues from unrelated entities									865,645,000	799,169,000	1,067,315,000
UNITED KINGDOM | United Kingdom Building Services
Segment Reporting Information [Line Items]
Revenues from unrelated entities									414,871,000	340,745,000	326,256,000
Operating Segments [Member] | UNITED STATES | United States electrical construction and facilities services segment
Segment Reporting Information [Line Items]
Revenues from unrelated entities									1,957,819,000
Total revenues									1,959,978,000	1,836,985,000	1,728,920,000
Operating income (loss)									139,430,000	150,001,000	101,761,000
Restructuring expenses									0	(452,000)
Capital expenditures									5,993,000	4,797,000	5,294,000
Depreciation and amortization of Property, plant and equipment									6,221,000	6,545,000	6,318,000
Contract assets	38,412,000				35,060,000				38,412,000	35,060,000	46,193,000
Contract liabilities	163,951,000				178,454,000				163,951,000	178,454,000	163,794,000
Long-lived assets	201,333,000				180,990,000				201,333,000	180,990,000	183,632,000
Total assets	702,112,000				617,471,000				702,112,000	617,471,000	631,581,000
Operating Segments [Member] | UNITED STATES | United States Mechanical Construction And Facilities Services
Segment Reporting Information [Line Items]
Revenues from unrelated entities									3,038,665,000
Total revenues									3,061,832,000	2,994,700,000	2,662,100,000
Operating income (loss)									219,352,000	212,396,000	132,743,000
Restructuring expenses									0	(180,000)
Capital expenditures									7,613,000	6,778,000	7,672,000
Depreciation and amortization of Property, plant and equipment									8,104,000	7,819,000	7,544,000
Contract assets	50,463,000				52,381,000				50,463,000	52,381,000	46,703,000
Contract liabilities	285,103,000				290,216,000				285,103,000	290,216,000	271,811,000
Long-lived assets	346,977,000				352,970,000				346,977,000	352,970,000	287,744,000
Total assets	1,113,417,000				1,097,240,000				1,113,417,000	1,097,240,000	954,633,000
Operating Segments [Member] | UNITED STATES | United States Building Services
Segment Reporting Information [Line Items]
Total revenues									1,942,663,000	1,812,763,000	1,864,824,000
Operating income (loss)									93,827,000	81,720,000	77,100,000
Restructuring expenses									(882,000)	(945,000)
Capital expenditures									10,414,000	10,745,000	11,080,000
Depreciation and amortization of Property, plant and equipment									10,324,000	11,051,000	10,241,000
Contract assets	33,304,000				26,028,000				33,304,000	26,028,000	28,084,000
Contract liabilities	79,281,000				48,481,000				79,281,000	48,481,000	50,546,000
Long-lived assets	436,887,000				409,718,000				436,887,000	409,718,000	401,154,000
Total assets	846,221,000				764,085,000				846,221,000	764,085,000	753,434,000
Operating Segments [Member] | UNITED STATES | United States Industrial Services
Segment Reporting Information [Line Items]
Total revenues									867,810,000	801,531,000	1,068,662,000
Operating income (loss)									28,172,000	19,084,000	77,845,000
Capital expenditures									18,277,000	9,583,000	10,065,000
Depreciation and amortization of Property, plant and equipment									9,642,000	10,274,000	10,394,000
Contract assets	12,956,000				1,636,000				12,956,000	1,636,000	2,572,000
Contract liabilities	8,050,000				3,098,000				8,050,000	3,098,000	1,823,000
Long-lived assets	617,991,000				630,184,000				617,991,000	630,184,000	709,267,000
Total assets	832,034,000				772,899,000				832,034,000	772,899,000	850,434,000
Operating Segments [Member] | UNITED KINGDOM | United Kingdom Building Services
Segment Reporting Information [Line Items]
Total revenues									414,871,000	340,745,000	326,256,000
Operating income (loss)									15,930,000	12,905,000	10,086,000
Capital expenditures									870,000	2,166,000	4,523,000
Depreciation and amortization of Property, plant and equipment									3,447,000	3,371,000	3,560,000
Contract assets	23,108,000				7,516,000				23,108,000	7,516,000	7,145,000
Contract liabilities	15,905,000				3,907,000				15,905,000	3,907,000	1,268,000
Long-lived assets	9,264,000				11,729,000				9,264,000	11,729,000	11,446,000
Total assets	146,379,000				131,806,000				146,379,000	131,806,000	105,081,000
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Operating income (loss)									(90,415,000)	(87,808,000)	(88,740,000)
Restructuring expenses									(1,424,000)	0
Capital expenditures									312,000	615,000	1,014,000
Depreciation and amortization of Property, plant and equipment									734,000	855,000	824,000
Long-lived assets	1,072,000				1,494,000				1,072,000	1,494,000	1,734,000
Total assets	$ 448,644,000				$ 582,403,000				448,644,000	582,403,000	557,275,000
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Total revenues									(116,523,000)	$ (99,725,000)	$ (99,238,000)
Intersegment Eliminations [Member] | UNITED STATES | United States electrical construction and facilities services segment
Segment Reporting Information [Line Items]
Revenues from unrelated entities									(3,496,000)
Intersegment Eliminations [Member] | UNITED STATES | United States Mechanical Construction And Facilities Services
Segment Reporting Information [Line Items]
Revenues from unrelated entities									$ (18,358,000)