Stockholders' Deficit - Components of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	$ (39.5)	$ (309.8)
Amounts reclassified from AOCL into earnings, net of tax	(4.4)	1.1
Ending balance	(149.7)	(374.2)
Investment Securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	24.7	(1.1)
Unrealized gains	7.2	16.8
Tax benefit/(expense)	(1.1)	(3.8)
Amounts reclassified from AOCL into earnings, net of tax	(0.4)
Ending balance	30.4	11.9
Hedging Activities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	(3.6)	7.4
Unrealized gains	27.3	4.4
Tax benefit/(expense)	(0.3)	0.9
Amounts reclassified from AOCL into earnings, net of tax	(6.3)	(1.4)
Ending balance	17.1	11.3
Foreign Currency Translation
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	(101.2)	(101.2)
Ending balance	(101.2)	(101.2)
Defined benefit plans
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	(128.9)	(136.1)
Amounts reclassified from AOCL into earnings, net of tax	2.3	2.5
Ending balance	(126.6)	(133.6)
Total AOCL
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	(209.0)	(231.0)
Unrealized gains	34.5	21.2
Tax benefit/(expense)	(1.4)	(2.9)
Amounts reclassified from AOCL into earnings, net of tax	(4.4)	1.1
Ending balance	$ (180.3)	$ (211.6)