Significant Business Transactions and Events - BFDS and IFDS U.K. Pro Forma Additional Information (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]
Net Income	$ 81.2	$ 48.5	$ 75.4	$ 246.4	$ 42.9	$ 273.3	$ 53.0	$ 58.1	$ 451.5	$ 427.3	$ 358.2
BFDS and IFDS U.K.
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]
Transaction costs	$ 5.1								5.1
BFDS and IFDS U.K. | Net Gains On Transaction And Related Tax Impact [Member]
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]
Net Income									$ 188.6