Segment Information (Information About Industry Segments And Geographic Areas) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Segment Reporting Information [Line Items]
Revenues from unrelated entities	$ 1,744,970	$ 1,589,187
Total revenues	1,744,970	1,589,187
Operating income (loss)	55,615	55,330
Restructuring expenses	(91)	(8)
Interest expense	(2,376)	(2,216)
Interest income	165	176
Income from continuing operations before income taxes	53,404	53,290
Total assets	3,449,659		$ 3,542,657
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Total revenues	(22,207)	(14,412)
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Operating income (loss)	(20,957)	(18,426)
Total assets	441,343		536,993
UNITED STATES
Segment Reporting Information [Line Items]
Revenues from unrelated entities	1,657,334	1,502,237
Total revenues	1,657,334	1,502,237
Operating income (loss)	73,352	71,386
Total assets	2,876,998		2,871,882
UNITED STATES | United States Electrical Construction And Facilities Services [Member]
Segment Reporting Information [Line Items]
Revenues from unrelated entities	348,289	318,994
UNITED STATES | United States Mechanical Construction And Facilities Services [Member]
Segment Reporting Information [Line Items]
Revenues from unrelated entities	611,890	511,026
UNITED STATES | United States Building Services [Member]
Segment Reporting Information [Line Items]
Revenues from unrelated entities	439,651	439,492
UNITED STATES | United States Industrial Services [Member]
Segment Reporting Information [Line Items]
Revenues from unrelated entities	257,504	232,725
UNITED STATES | Operating Segments [Member] | United States Electrical Construction And Facilities Services [Member]
Segment Reporting Information [Line Items]
Total revenues	354,104	320,238
Operating income (loss)	16,707	16,674
Restructuring expenses	0	(2)
Total assets	365,229		372,525
UNITED STATES | Operating Segments [Member] | United States Mechanical Construction And Facilities Services [Member]
Segment Reporting Information [Line Items]
Total revenues	615,232	513,367
Operating income (loss)	23,877	20,901
Restructuring expenses	0	(6)
Total assets	858,499		894,366
UNITED STATES | Operating Segments [Member] | United States Building Services [Member]
Segment Reporting Information [Line Items]
Total revenues	452,335	449,728
Operating income (loss)	13,902	20,978
Restructuring expenses	(91)	0
Total assets	724,330		721,653
UNITED STATES | Operating Segments [Member] | United States Industrial Services [Member]
Segment Reporting Information [Line Items]
Total revenues	257,870	233,316
Operating income (loss)	18,866	12,833
Total assets	928,940		883,338
UNITED KINGDOM | United Kingdom Building Services [Member]
Segment Reporting Information [Line Items]
Revenues from unrelated entities	87,636	86,950
UNITED KINGDOM | Operating Segments [Member] | United Kingdom Building Services [Member]
Segment Reporting Information [Line Items]
Total revenues	87,636	86,950
Operating income (loss)	3,311	$ 2,378
Total assets	$ 131,318		$ 133,782