Accumulated Other Comprehensive Income (Loss) (Summary of Changes in Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance			$ 10,508.3
Net Change	$ 185.5	$ (2.4)	293.9	$ (68.5)
Ending balance	10,805.5	10,357.6	10,805.5	10,357.6
Net Unrealized Gains (Losses) on Debt Securities Available for Sale
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance			(114.9)	(74.8)
Net Change	162.4	(19.9)	248.0	(74.9)
Reclassification of Certain Tax Effects from AOCI				(17.8)
Ending balance	133.1	(167.5)	133.1	(167.5)
Net Unrealized Gains (Losses) on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance			4.0	4.5
Net Change	4.8	(3.5)	0.5	(7.2)
Reclassification of Certain Tax Effects from AOCI				0.9
Ending balance	4.5	(1.8)	4.5	(1.8)
Net Foreign Currency Adjustments
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance			67.9	(1.8)
Net Change	14.1	13.6	28.9	0.5
Reclassification of Certain Tax Effects from AOCI				47.5
Ending balance	96.8	46.2	96.8	46.2
Net Pension and Other Postretirement Benefit Adjustments
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance			(410.7)	(342.2)
Net Change	4.2	7.4	16.5	13.1
Reclassification of Certain Tax Effects from AOCI				(55.9)
Ending balance	(394.2)	(385.0)	(394.2)	(385.0)
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance			(453.7)	(414.3)
Net Change			293.9	(68.5)
Reclassification of Certain Tax Effects from AOCI			0.0	(25.3)
Ending balance	$ (159.8)	$ (508.1)	$ (159.8)	$ (508.1)