Goodwill and Other Intangible Assets (Schedule of Consideration Paid and Net Assets Acquired, Business Combinations) (Details) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Acquisition consideration:
Fair value of contingent consideration (earn-out liability)	$ 5,711	$ 89,614
Identifiable assets acquired and liabilities assumed:
Goodwill	$ 1,149,795		$ 1,137,738
2017 Acquisitions [Member]
Acquisition consideration:
Cash			120,400
Fair value of contingent consideration (earn-out liability)			98,500
Total consideration transferred			218,900
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			(29,500)
Long-term debt, including capital lease obligations			(9,900)
Deferred income taxes and other long-term liabilities			(12,000)
Total identifiable net assets			77,700
Goodwill			141,200
Total net assets acquired, including goodwill			$ 218,900