Income Taxes - Schedule of Components of Provision for Income Taxes (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current provision:
Federal	$ 29.5	$ 33.6	$ 33.3
State and local	3.0	3.3	3.2
Current income tax expense (benefit)	32.5	36.9	36.5
Deferred (benefit) provision:
Federal	(53.0)	12.7	6.0
State and local	0.8	1.1	2.1
Foreign	(0.2)	0.0	0.0
Deferred income tax expense (benefit)	(52.4)	13.8	8.1
Income tax expense (benefit)	$ (19.9)	$ 50.7	$ 44.6