Independent Investigation of the Audit Committee and Related Restatements (Narrative) (Details) $ / shares in Units, $ in Thousands		3 Months Ended																6 Months Ended			9 Months Ended			12 Months Ended
		Dec. 31, 2014 USD ($) $ / shares		Sep. 30, 2014 USD ($) $ / shares		Jun. 30, 2014 USD ($) $ / shares		Mar. 31, 2014 USD ($) $ / shares		Dec. 31, 2013 USD ($) $ / shares		Sep. 30, 2013 USD ($) $ / shares		Jun. 30, 2013 USD ($) $ / shares		Mar. 31, 2013 USD ($) $ / shares		Jun. 30, 2014 USD ($) $ / shares		Jun. 30, 2013	Sep. 30, 2014 USD ($) $ / shares		Sep. 30, 2013	Dec. 31, 2014 USD ($) $ / shares	Dec. 31, 2013 USD ($) $ / shares	Dec. 31, 2012 USD ($) $ / shares
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net Income		$ 21,100	[1]	$ 48,986		$ 33,725		$ 12,097		$ 41,500		$ 46,100		$ 34,900		$ 18,400		$ 45,821			$ 94,807			$ 115,923	$ 140,950	$ 107,426
Revenue		1,231,300	[1]	1,315,488		1,107,232		957,818		1,159,100		1,269,400		977,600		918,600		2,065,050			3,380,538			4,611,803	4,324,787	3,726,789
Costs of revenue, excluding depreciation and amortization		$ 1,063,100	[1]	1,122,861		950,715		841,324		$ 987,000		$ 1,081,100		$ 822,700		$ 791,500		1,792,040			2,914,901			3,977,963	3,682,367	3,239,195
Increase (decrease), other income, net				$ 1,416		$ 1,923		$ 2,083										$ 4,007			$ 5,424			$ 8,116	$ 6,188	$ (8,017)
Basic earnings per share, continuing operations (in dollars per share) | $ / shares		$ 0.33	[1]	$ 0.60		$ 0.43		$ 0.16		$ 0.55		$ 0.65		$ 0.46		$ 0.25		$ 0.59			$ 1.21			$ 1.53	$ 1.92	$ 1.49
Diluted earnings per share, continuing operations (in dollars per share) | $ / shares		0.32	[1]	0.57		0.39		0.14		0.50		0.59		0.42		0.23		0.53			1.11			1.42	1.74	1.42
Total basic earnings per share (in dollars per share) | $ / shares		0.26	[1],[2]	0.60	[2],[3]	0.43	[2],[3]	0.16	[2],[3]	0.54	[2]	0.60	[2]	0.46	[2]	0.24	[2]	0.59	[3]		1.20	[3]		1.45	1.83	1.37
Total diluted earnings per share (in dollars per share) | $ / shares		$ 0.25	[1],[2]	$ 0.57	[2],[3]	$ 0.39	[2],[3]	$ 0.14	[2],[3]	$ 0.49	[2]	$ 0.54	[2]	$ 0.41	[2]	$ 0.22	[2]	$ 0.53	[3]		$ 1.10	[3]		$ 1.35	$ 1.66	$ 1.31
EBITDA				$ 134,200		$ 104,200		$ 65,300										$ 169,500			$ 303,600			$ 403,700	$ 427,600	$ 322,100
Increase (decrease) in revenue, amount, acquisition																								565,400	406,600	$ 170,800
Cost and earnings in excess of billings		$ 485,600								$ 392,900														485,600	392,900
Other current liabilities		66,527		17,665		13,890		20,431		26,162								13,890			17,665			66,527	26,162
Acquisition-related contingent consideration, net of current portion		103,515		115,649		116,929		112,950		112,370								116,929			115,649			103,515	112,370
Acquisition-related contingent consideration, current		49,798		39,126		36,479		71,500		67,226								36,479			39,126			49,798	67,226
Billings in excess of costs and earnings		$ 155,674		129,732		108,968		109,370		$ 121,641								108,968			129,732			$ 155,674	$ 121,641
Electrical Transmission [Member]
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Revenue				138,000		117,000		74,000										191,000			329,000
EBITDA				$ 19,000		$ 19,000		$ (3,000)										$ 17,000			$ 35,000
EBITDA margin				13.40%		16.50%		(3.70%)				10.20%		9.70%		4.00%		8.70%		7.30%	10.70%		8.40%
Revenue, increase (decrease), period over period, amount				$ 20,000		$ (2,000)		$ (11,000)										$ (12,000)			$ 7,000
Revenue, increase (decrease), period over period, percent				16.60%		(1.50%)		(13.00%)										(6.10%)			2.30%
Increase (decrease) in revenue, amount, acquisition				$ 3,000		$ 8,000		$ 5,000										$ 13,000			$ 16,000
Increase (decrease) in revenue, amount, organic				17,000		(10,000)		(15,000)										(25,000)			(9,000)
EBITDA, period over period, increase (decrease), amount				$ 6,000		$ 8,000		$ (6,000)										$ 2,000			$ 8,000
EBITDA, period over period, increase (decrease), percent				53.30%		67.70%		(180.00%)										11.20%			30.00%
EBITDA margin, period over period, increase (decrease), percent				3.20%		6.80%												1.40%			2.30%
EBITDA margin, period over period, increase (decrease), amount				$ 4,000		$ 8,000												$ 3,000			$ 8,000
Adjustments [Member]
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net Income				3,715		1,675		$ (3,926)										(2,252)			1,463
Revenue				5,892		2,676		(6,211)										(3,535)			2,358
Costs of revenue, excluding depreciation and amortization				(100)		(174)		270										97			(3)
Increase (decrease), other income, net				$ 0		$ (128)		$ 128										$ 0			$ 0
Basic earnings per share, continuing operations (in dollars per share) | $ / shares				$ 0.04		$ 0.02		$ (0.05)										$ (0.03)			$ 0.02
Diluted earnings per share, continuing operations (in dollars per share) | $ / shares				0.04		0.02		(0.05)										(0.03)			0.02
Total basic earnings per share (in dollars per share) | $ / shares	[3]			0.05		0.02		(0.05)										(0.03)			0.02
Total diluted earnings per share (in dollars per share) | $ / shares				$ 0.04	[3]	$ 0.02	[3]	$ (0.05)	[3]									$ (0.03)			$ 0.02	[3]
EBITDA				$ 6,000		$ 2,700		$ (6,400)										$ (3,600)			$ 2,400
Other current liabilities				(5,311)		(4,050)		(4,157)										(4,050)			(5,311)
Current taxes receivable						1,400		2,400										1,400
Acquisition-related contingent consideration, net of current portion				0		0		(6,806)										0			0
Acquisition-related contingent consideration, current				0		0		6,806										0			0
Billings in excess of costs and earnings				(1,265)		(837)		0										(837)			(1,265)
Current taxes payable				900																	900
Adjustments [Member] | Reclassification Adjustment [Member]
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Acquisition-related contingent consideration, net of current portion								(6,800)
Acquisition-related contingent consideration, current								6,800
Adjustments [Member] | Contracts Accounted for under Percentage of Completion [Member]
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Cost and earnings in excess of billings				(4,200)																	(4,200)
Billings in excess of costs and earnings				(1,300)		(800)												(800)			(1,300)
Adjustments [Member] | Contracts Accounted for under Percentage of Completion [Member] | Reclassification Adjustment [Member]
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Cost and earnings in excess of billings				(5,300)		(4,100)		(4,200)										(4,100)			(5,300)
Other current liabilities				$ (5,300)		$ (4,100)		$ (4,200)										$ (4,100)			$ (5,300)
Adjustments [Member] | Contracts Accounted for under Percentage of Completion [Member] | Electrical Transmission [Member]
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Number of projects				2		2		2										2			2			2
Net Income				$ 3,600		$ 1,400		$ (3,800)
Revenue				5,800		2,300												$ (3,900)			$ 1,900
Cost and earnings in excess of billings				1,100		(4,400)		(6,200)										(4,400)			1,100
As Reported [Member]
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net Income				45,271		32,050		16,023										48,073			93,344
Revenue				1,309,596		1,104,556		964,029										2,068,585			3,378,180
Costs of revenue, excluding depreciation and amortization				1,122,961		950,889		841,054										1,791,943			2,914,904
Increase (decrease), other income, net				$ 1,416		$ 2,051		$ 1,955										$ 4,007			$ 5,424
Basic earnings per share, continuing operations (in dollars per share) | $ / shares				$ 0.56		$ 0.41		$ 0.21										$ 0.62			$ 1.19
Diluted earnings per share, continuing operations (in dollars per share) | $ / shares				0.53		0.37		0.19										0.56			1.09
Total basic earnings per share (in dollars per share) | $ / shares	[3]			0.55		0.41		0.21										0.62			1.18
Total diluted earnings per share (in dollars per share) | $ / shares	[3]			$ 0.53		$ 0.37		$ 0.19										$ 0.56			$ 1.08
EBITDA				$ 128,200		$ 101,500		$ 71,600										$ 173,100			$ 301,200
Other current liabilities				22,976		17,940		24,588										17,940			22,976
Acquisition-related contingent consideration, net of current portion				115,649		116,929		119,756										116,929			115,649
Acquisition-related contingent consideration, current				39,126		36,479		64,694										36,479			39,126
Billings in excess of costs and earnings				130,997		109,805		109,370										109,805			130,997
As Reported [Member] | Electrical Transmission [Member]
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Revenue				133,000		114,000		80,000										195,000			327,000
EBITDA				$ 13,000		$ 17,000		$ 3,000										$ 21,000			$ 33,000
EBITDA margin				9.60%		14.90%		4.40%										10.50%			10.10%

[1] Preliminary, unaudited fourth quarter and full year 2014 results were previously issued pursuant to a press release dated February 26, 2015 and not previously filed in a report under the Securities Exchange Act of 1934, as amended. The unaudited quarterly information for the three month period ended December 31, 2014 in the table above includes the effect of adjustments determined by management following the review process described in Note 2 - Independent Investigation of the Audit Committee and Related Restatements.

[2] Earnings per share calculations, including the sum of the individual quarterly earnings per share amounts to the full year earnings per share amounts, may contain slight summation differences due to rounding.

[3] Earnings per share calculations may contain slight summation differences due to rounding.