Intangible Franchise Rights and Goodwill (Details) - USD ($)		3 Months Ended
		Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Indefinite-lived Intangible Assets [Line Items]
Indefinite-lived Intangible Assets Acquired		$ 8,755,000
Intangible Franchise Rights [Roll Forward]
Intangible Franchise Rights, Beginning Balance		285,632,000
Currency Translation		1,239,000
Intangible Franchise Rights, Ending Balance		295,626,000
Asset Impairment Charges		0
Goodwill [Roll Forward]
Goodwill, Beginning Balance		913,034,000
Goodwill, Acquired During Period		44,129,000
Goodwill, Purchase Accounting Adjustments		18,000
Goodwill, Foreign Currency Translation Gain (Loss)		2,929,000
Goodwill, Ending Balance	[1]	960,110,000
Goodwill, Impaired, Accumulated Impairment Loss			$ 97,800,000	$ 97,800,000
BRAZIL
Indefinite-lived Intangible Assets [Line Items]
Indefinite-lived Intangible Assets Acquired		0
Intangible Franchise Rights [Roll Forward]
Intangible Franchise Rights, Beginning Balance		168,000
Currency Translation		1,000
Intangible Franchise Rights, Ending Balance		169,000
Goodwill [Roll Forward]
Goodwill, Beginning Balance	[1]	12,733,000
Goodwill, Acquired During Period		0
Goodwill, Purchase Accounting Adjustments		0
Goodwill, Foreign Currency Translation Gain (Loss)		28,000
Goodwill, Ending Balance	[1]	12,761,000
UNITED KINGDOM
Indefinite-lived Intangible Assets [Line Items]
Indefinite-lived Intangible Assets Acquired		7,454,000
Intangible Franchise Rights [Roll Forward]
Intangible Franchise Rights, Beginning Balance		29,483,000
Currency Translation		1,238,000
Intangible Franchise Rights, Ending Balance		38,175,000
Goodwill [Roll Forward]
Goodwill, Beginning Balance	[1]	65,034,000
Goodwill, Acquired During Period		29,930,000
Goodwill, Purchase Accounting Adjustments		0
Goodwill, Foreign Currency Translation Gain (Loss)		2,901,000
Goodwill, Ending Balance	[1]	97,865,000
UNITED STATES
Indefinite-lived Intangible Assets [Line Items]
Indefinite-lived Intangible Assets Acquired		1,301,000
Intangible Franchise Rights [Roll Forward]
Intangible Franchise Rights, Beginning Balance		255,981,000
Currency Translation		0
Intangible Franchise Rights, Ending Balance		257,282,000
Goodwill [Roll Forward]
Goodwill, Beginning Balance	[1]	835,267,000
Goodwill, Acquired During Period		14,199,000
Goodwill, Purchase Accounting Adjustments		18,000
Goodwill, Foreign Currency Translation Gain (Loss)		0
Goodwill, Ending Balance	[1]	$ 849,484,000

[1] (1) Net of accumulated impairment of $97.8 million