Income Taxes - Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at January 1	$ 43.2	$ 13.1
Acquired uncertain tax positions	0.0	28.5
Unrecognized Tax Benefits, Increase Resulting from Prior Period Tax Positions	6.1	6.3
Increases related to current year tax positions	7.5	2.5
Settlements	(1.8)	(3.4)
Reductions due to lapse of applicable statute of limitations	(6.6)	(3.8)
Balance at December 31	$ 48.4	$ 43.2