Stockholders' Equity - Components of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	$ 286.3	$ 218.8	$ 186.6	$ (73.4)	$ (149.7)	$ (39.5)	$ 186.6	$ (39.5)
Amounts reclassified from AOCL into earnings, net of tax	0.9			6.0			15.0	(0.7)
Ending balance	368.6	286.3	218.8	67.1	(73.4)	(149.7)	368.6	67.1
Investment Securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	47.5	45.3	58.3	49.9	30.4	24.7	58.3	24.7
Unrealized gains/(losses)	(7.3)	2.3	(16.0)	8.5	24.6	7.2
Tax benefit/(expense)	1.4	(0.2)	2.9	(1.4)	(4.8)	(1.1)
Amounts reclassified from AOCL into earnings, net of tax	(2.0)	0.1	0.1		(0.3)	(0.4)	(1.8)	(0.7)
Ending balance	39.6	47.5	45.3	57.0	49.9	30.4	39.6	57.0
Hedging Activities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	(4.3)	(1.6)	(30.5)	(2.0)	17.1	(3.6)	(30.5)	(3.6)
Unrealized gains/(losses)	17.7	(6.2)	24.2	(22.1)	(15.1)	27.3
Tax benefit/(expense)	(0.1)		(0.9)	0.2	0.1	(0.3)
Amounts reclassified from AOCL into earnings, net of tax	0.6	3.5	5.6	3.7	(4.1)	(6.3)	9.7	(6.7)
Ending balance	13.9	(4.3)	(1.6)	(20.2)	(2.0)	17.1	13.9	(20.2)
Foreign Currency Translation
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	(101.2)	(101.2)	(101.2)	(101.2)	(101.2)	(101.2)	(101.2)	(101.2)
Ending balance	(101.2)	(101.2)	(101.2)	(101.2)	(101.2)	(101.2)	(101.2)	(101.2)
Defined benefit plans
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	(81.3)	(83.6)	(86.1)	(124.5)	(126.6)	(128.9)	(86.1)	(128.9)
Amounts reclassified from AOCL into earnings, net of tax	2.3	2.3	2.5	2.3	2.1	2.3	7.1	6.7
Ending balance	(79.0)	(81.3)	(83.6)	(122.2)	(124.5)	(126.6)	(79.0)	(122.2)
Total AOCL
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	(139.3)	(141.1)	(159.5)	(177.8)	(180.3)	(209.0)	(159.5)	(209.0)
Unrealized gains/(losses)	10.4	(3.9)	8.2	(13.6)	9.5	34.5
Tax benefit/(expense)	1.3	(0.2)	2.0	(1.2)	(4.7)	(1.4)
Amounts reclassified from AOCL into earnings, net of tax	0.9	5.9	8.2	6.0	(2.3)	(4.4)
Ending balance	$ (126.7)	$ (139.3)	$ (141.1)	$ (186.6)	$ (177.8)	$ (180.3)	$ (126.7)	$ (186.6)