Acquisitions (Schedule of Consideration Paid and Net Assets Acquired, Business Acquisitions - Big Country) (Details) - USD ($) $ in Thousands		3 Months Ended	6 Months Ended	9 Months Ended	12 Months Ended
	May. 01, 2013	Mar. 31, 2014	Jun. 30, 2014	Sep. 30, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Identifiable assets acquired and liabilities assumed:
Goodwill		$ 912,885	$ 983,133	$ 1,000,024	$ 1,082,466	$ 902,044	$ 826,600
Earn-Out Arrangements [Member]
Acquisition consideration:
Fair value of contingent consideration (earn-out liability)		$ 8,700	$ 33,612	$ 34,988	$ 33,612	$ 32,451	$ 66,700
Big Country [Member] | Canadian Dollars [Member]
Acquisition consideration:
Cash	$ 103,500
Total consideration transferred	128,800
Identifiable assets acquired and liabilities assumed:
Current assets	69,000
Property and equipment	43,500
Finite-lived intangible assets	10,700
Current liabilities, including current portion of capital lease obligations and long-term debt	(24,400)
Long-term debt, including capital lease obligations	(23,000)
Deferred income taxes	(14,400)
Total identifiable net assets	91,000
Goodwill	37,800
Total net assets acquired, including goodwill	128,800
Big Country [Member] | Canadian Dollars [Member] | Pre-Qualifications [Member]
Identifiable assets acquired and liabilities assumed:
Pre-qualifications	29,600
Big Country [Member] | Canadian Dollars [Member] | Earn-Out Arrangements [Member]
Acquisition consideration:
Fair value of contingent consideration (earn-out liability)	$ 25,300