Income Taxes - Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits, beginning balance	$ 128.3	$ 112.2	$ 27.7
Additions based on current year tax positions	6.5	76.2	20.7
Additions based upon prior year tax positions (including acquired uncertain tax positions)	4.0	1.7	72.2
Reductions due to closed statutes	(6.3)	(9.9)	(2.9)
Reductions for prior period positions	(38.0)	(51.9)	0.0
Reductions for settlements and payments	(4.2)	0.0	(5.5)
Unrecognized tax benefits, ending balance	$ 90.3	$ 128.3	$ 112.2