Stockholders' Equity - Components of Accumulated Other Comprehensive Income, Net of Tax (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 6,678.8
Other comprehensive income (loss) before reclassifications	30.9	$ 1.7
Reclassified from accumulated other comprehensive (loss)	(5.2)	0.5
Other comprehensive gains (losses)	25.7	2.2	$ (6.6)
Ending balance	8,263.8	6,678.8
Gains (Losses) on Hedge Instruments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	0.2	(2.4)
Other comprehensive income (loss) before reclassifications	5.8	3.6
Reclassified from accumulated other comprehensive (loss)	(5.3)	(1.0)
Other comprehensive gains (losses)	0.5	2.6
Ending balance	0.7	0.2	(2.4)
Unrealized  Gains (Losses) on Available-for-Sale Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(9.8)	(11.3)
Other comprehensive income (loss) before reclassifications	30.7	0.3
Reclassified from accumulated other comprehensive (loss)	(0.5)	1.2
Other comprehensive gains (losses)	30.2	1.5
Ending balance	20.4	(9.8)	(11.3)
Foreign Currency Translation Gains (Losses)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(0.3)	2.3
Other comprehensive income (loss) before reclassifications	0.3	(2.6)
Reclassified from accumulated other comprehensive (loss)	0.0	0.0
Other comprehensive gains (losses)	0.3	(2.6)
Ending balance	0.0	(0.3)	2.3
Employee Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(3.4)	(4.1)
Other comprehensive income (loss) before reclassifications	(5.9)	0.4
Reclassified from accumulated other comprehensive (loss)	0.6	0.3
Other comprehensive gains (losses)	(5.3)	0.7
Ending balance	(8.7)	(3.4)	(4.1)
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(13.3)	(15.5)
Ending balance	$ 12.4	$ (13.3)	$ (15.5)