Income Taxes - Net deferred tax assets (liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Differences in capitalization and depreciation and amortization of reacquired franchises and equipment	$ 0.0	$ 4.7
Differences in acquisition financing costs	0.1	0.3
Employee compensation	7.6	12.6
Deferred gain on sale of assets	0.7	5.5
Book/tax difference in revenue recognition	14.2	45.8
Other	21.8	35.4
Deferred tax assets	44.4	104.3
Valuation allowance	0.0	(1.1)
Total deferred tax assets after valuation allowance	44.4	103.2
Differences between financial and tax accounting in the recognition of franchise and equipment sales	(20.7)	(39.1)
Differences in capitalization and depreciation (1)	(147.5)	(281.3)
Book/tax difference in revenue recognition	(2.4)	(7.8)
Differences between book and tax basis of property and equipment	(8.5)	(14.6)
Other	(3.5)	(14.3)
Deferred tax liabilities	(182.6)	(357.1)
Net deferred tax liabilities	$ 138.2	$ 253.9