Goodwill and Other Intangible Assets (Schedule of Consideration Paid and Net Assets Acquired, Business Combinations) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Identifiable assets acquired and liabilities assumed:
Goodwill	$ 1,137,738	$ 995,874	$ 988,500
Earn-Out Liabilities [Member]
Acquisition consideration:
Fair value of contingent consideration (earn-out liability)	102,534	$ 0	$ 0
2017 Acquisitions [Member]
Acquisition consideration:
Cash	117,900
Total consideration transferred	211,400
Identifiable assets acquired and liabilities assumed:
Current assets, primarily composed of accounts receivable and $2.8 million of cash acquired	42,700
Cash acquired	2,800
Property and equipment	56,700
Amortizing intangible assets	28,800
Other long-term assets	500
Current liabilities, including current portion of capital lease obligations and long-term debt	(29,200)
Long-term debt, including capital lease obligations	(9,900)
Deferred income taxes	(16,200)
Total identifiable net assets	73,400
Goodwill	138,000
Total net assets acquired, including goodwill	211,400
2017 Acquisitions [Member] | Earn-Out Liabilities [Member]
Acquisition consideration:
Fair value of contingent consideration (earn-out liability)	$ 93,500