Intangibles - Amortizable Intangible Assets Other Than Goodwill (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Goodwill and Intangible Assets Disclosure [Abstract]
Noncompetition and other agreements	$ 429,140	$ 407,220
Lease agreements	7,623	7,244
Indefinite-lived assets	33,255	1,125
Other	583	583
Intangible assets, gross (excluding goodwill)	470,601	416,172
Less accumulated amortization	(356,774)	(342,668)
Total intangible assets	$ 113,827	$ 73,504