Segment Information (Information About Industry Segments And Geographic Areas) (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Segment Reporting Information [Line Items]
Revenues from unrelated entities	$ 1,895,937	$ 1,933,416	$ 3,787,669	$ 3,678,386
Total revenues	1,895,937	1,933,416	3,787,669	3,678,386
Operating income (loss)	92,821	92,289	175,595	147,904
Restructuring expenses	(343)	(641)	(908)	(732)
Interest expense	(3,069)	(3,118)	(6,140)	(5,494)
Interest income	73	192	330	357
Income from continuing operations before income taxes	89,825	89,363	169,785	142,767
Total assets	3,847,574		3,847,574		$ 3,852,438
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Total revenues	(26,246)	(26,249)	(50,788)	(48,456)
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Operating income (loss)	(20,063)	(22,958)	(41,123)	(43,915)
Total assets	489,777		489,777		557,275
UNITED STATES
Segment Reporting Information [Line Items]
Revenues from unrelated entities	1,816,779	1,842,832	3,629,493	3,500,166
Total revenues	1,816,779	1,842,832	3,629,493	3,500,166
Operating income (loss)	109,730	112,630	212,450	185,982
Total assets	3,245,020		3,245,020		3,190,082
UNITED STATES \| United States Electrical Construction And Facilities Services [Member]
Segment Reporting Information [Line Items]
Revenues from unrelated entities	449,222	420,632	892,238	768,921
UNITED STATES \| United States Mechanical Construction And Facilities Services [Member]
Segment Reporting Information [Line Items]
Revenues from unrelated entities	741,817	625,547	1,412,946	1,233,975
UNITED STATES \| United States Building Services [Member]
Segment Reporting Information [Line Items]
Revenues from unrelated entities	438,264	463,145	878,294	906,258
UNITED STATES \| United States Industrial Services [Member]
Segment Reporting Information [Line Items]
Revenues from unrelated entities	187,476	333,508	446,015	591,012
UNITED STATES \| Operating Segments [Member] \| United States Electrical Construction And Facilities Services [Member]
Segment Reporting Information [Line Items]
Total revenues	451,124	430,101	895,140	784,205
Operating income (loss)	32,118	23,011	63,152	39,718
Total assets	588,331		588,331		631,581
UNITED STATES \| Operating Segments [Member] \| United States Mechanical Construction And Facilities Services [Member]
Segment Reporting Information [Line Items]
Total revenues	750,442	628,770	1,429,833	1,240,540
Operating income (loss)	53,054	37,936	93,487	61,717
Restructuring expenses			(218)	(198)
Total assets	1,038,760		1,038,760		954,633
UNITED STATES \| Operating Segments [Member] \| United States Building Services [Member]
Segment Reporting Information [Line Items]
Total revenues	453,849	476,458	908,793	932,255
Operating income (loss)	20,185	18,535	34,394	32,533
Restructuring expenses			(690)	(534)
Total assets	784,941		784,941		753,434
UNITED STATES \| Operating Segments [Member] \| United States Industrial Services [Member]
Segment Reporting Information [Line Items]
Total revenues	187,610	333,752	446,515	591,622
Operating income (loss)	4,373	33,148	21,417	52,014
Total assets	832,988		832,988		850,434
UNITED KINGDOM \| United Kingdom Building Services [Member] [Member]
Segment Reporting Information [Line Items]
Revenues from unrelated entities	79,158	90,584	158,176	178,220
UNITED KINGDOM \| Operating Segments [Member] \| United Kingdom Building Services [Member] [Member]
Segment Reporting Information [Line Items]
Total revenues	79,158	90,584	158,176	178,220
Operating income (loss)	3,497	$ 3,258	5,176	$ 6,569
Total assets	$ 112,777		$ 112,777		$ 105,081

Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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