Income Taxes - Reconciliation of Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2014	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2012
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at beginning of year		$ 12.5	$ 16.6	$ 16.6
Additions based on tax positions related to the current year		0.0	0.8	0.2
Additions for tax positions of prior years	$ 1.0	0.2			$ 0.0
Reductions for tax positions of prior years		(0.4)	0.0		$ 0.0
Reductions for equity adjustment	$ (4.3)	0.0		0.0
Reductions for lapse of statute		0.0	(1.6)	(0.2)
Balance at end of year		$ 12.3	$ 12.5	$ 16.6