Income Taxes - Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Gross unrecognized tax benefits, beginning of year	$ 46.1	$ 23.8	$ 19.0
Increases in prior year tax positions	11.4	27.1	4.1
Decreases in prior year tax positions	(1.8)	(1.2)	(1.7)
Increases for current year tax positions	5.9	6.6	4.1
Expiration of statute of limitations and audit settlements	(29.4)	(10.2)	(1.7)
Gross unrecognized tax benefits, end of year	32.2	46.1	23.8
Potential interest and penalties	2.0	20.4	20.2
Balance, end of year	$ 34.2	$ 66.5	$ 44.0