Equity - Components of Accumulated Other Comprehensive (Loss) Income, Net of Tax (Details) $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance	$ 4,823.2
Balance	4,851.2
Accumulated Other Comprehensive (Loss) Income
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance	(18.2)
Other comprehensive income before reclassifications	6.1
Amount reclassified from accumulated other comprehensive loss	1.2
Other comprehensive income, net	7.3
Balance	(10.9)
Unrealized Gains/Losses on Available-for- Sale Debt Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance	25.5
Other comprehensive income before reclassifications	1.8
Amount reclassified from accumulated other comprehensive loss	0.0
Other comprehensive income, net	1.8
Balance	27.3
Unrealized Gains/Losses on Cash Flow Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance	(0.9)
Other comprehensive income before reclassifications	2.1
Amount reclassified from accumulated other comprehensive loss	1.2
Other comprehensive income, net	3.3
Balance	2.4
Foreign Currency Translation Adjustments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance	(42.8)
Other comprehensive income before reclassifications	2.2
Amount reclassified from accumulated other comprehensive loss	0.0
Other comprehensive income, net	2.2
Balance	$ (40.6)