Goodwill and Other Intangible Assets (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014
Goodwill [Line Items]
Goodwill, accumulated impairment loss	$ 0
Goodwill	1,082,466,000	$ 902,044,000	$ 826,600,000	$ 1,000,024,000	$ 983,133,000	$ 912,885,000
Amortization expense	25,100,000	21,200,000	12,200,000
Communications [Member]
Goodwill [Line Items]
Goodwill	417,700,000	326,800,000	306,100,000
Oil and Gas [Member]
Goodwill [Line Items]
Goodwill	397,300,000	307,700,000	273,400,000
Electrical Transmission [Member]
Goodwill [Line Items]
Goodwill	$ 149,900,000	149,900,000	$ 129,500,000
2013 Acquisitions [Member] | Measurement Period Adjustments [Member]
Goodwill [Line Items]
Goodwill		2,600,000
2013 Acquisitions [Member] | Measurement Period Adjustments [Member] | Communications [Member]
Goodwill [Line Items]
Goodwill		(900,000)
2013 Acquisitions [Member] | Measurement Period Adjustments [Member] | Oil and Gas [Member]
Goodwill [Line Items]
Goodwill		2,500,000
2013 Acquisitions [Member] | Measurement Period Adjustments [Member] | Electrical Transmission [Member]
Goodwill [Line Items]
Goodwill		$ 1,000,000