Goodwill and Other Intangible Assets (Schedule of Consideration Paid and Net Assets Acquired, Business Combinations) (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Identifiable assets acquired and liabilities assumed:
Goodwill	$ 1,145,434		$ 1,137,738
Earn-Out Liabilities [Member]
Acquisition consideration:
Fair value of contingent consideration (earn-out liability)	$ 4,161	$ 0
2017 Acquisitions [Member]
Acquisition consideration:
Cash			120,300
Total consideration transferred			216,600
Identifiable assets acquired and liabilities assumed:
Current assets, primarily composed of accounts receivable and $2.8 million of cash acquired			43,800
Cash acquired			2,800
Property and equipment and other long-term assets			57,200
Amortizing intangible assets			28,100
Current liabilities, including current portion of capital lease obligations and long-term debt			(29,400)
Long-term debt, including capital lease obligations			(9,900)
Deferred income taxes and other long-term liabilities			(10,600)
Total identifiable net assets			79,200
Goodwill			137,400
Total net assets acquired, including goodwill			216,600
2017 Acquisitions [Member] | Earn-Out Liabilities [Member]
Acquisition consideration:
Fair value of contingent consideration (earn-out liability)			$ 96,300