Income Taxes - Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Unrecognized Tax Benefits
Unrecognized tax benefit, if recognized, would have an impact on effective tax rate	$ 21.8
Decrease in unrecognized tax benefits is reasonably possible	4.3
Payment of interest and penalties accrued	1.3	$ 1.0
Activity for unrecognized tax benefits [Roll Forward]
Balance at beginning of period	27.4	10.8	$ 13.9
Tax positions related to current year additions	2.3	8.4	2.0
Additions for tax positions of prior years	0.0	9.7	2.4
Tax positions related to prior years reductions	(7.3)	(0.3)	(3.0)
Reductions due to lapse of statute of limitations on tax positions	0.0	(0.4)	(0.1)
Settlements	(0.6)	(0.8)	(4.4)
Balance at end of period	$ 21.8	$ 27.4	$ 10.8