Income Taxes - Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017
Unrecognized Tax Benefits
Unrecognized tax benefits	$ 21.8	$ 27.4	$ 10.8	$ 33.8	$ 21.8
Unrecognized tax benefit, if recognized, would have an impact on effective tax rate				33.8
Decrease in unrecognized tax benefits is reasonably possible				4.3
Payment of interest and penalties accrued				$ 1.7	$ 1.3
Activity for unrecognized tax benefits [Roll Forward]
Balance at beginning of period	21.8	27.4	10.8
Tax positions related to current year additions	10.8	2.3	8.4
Additions for tax positions of prior years	2.6	0.0	9.7
Tax positions related to prior years reductions	(0.2)	(7.3)	(0.3)
Reductions due to lapse of statute of limitations on tax positions	(1.2)	0.0	(0.4)
Settlements	0.0	(0.6)	(0.8)
Balance at end of period	$ 33.8	$ 21.8	$ 27.4