Income Taxes - Schedule of 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]
Balance as of January 1	$ 146.2	$ 117.2	$ 116.6
Gross increases related to current year tax positions	51.4	49.0	45.6
Gross increases (decreases) related to prior year tax positions	5.6	(5.6)	(42.3)
Gross decrease due to settlements, recapture, filed returns, and lapse of statutes of limitation	(13.7)	(14.4)	(2.7)
Balance as of December 31	$ 189.5	$ 146.2	$ 117.2