Accumulated Other Comprehensive Income (Loss) (Summary of Changes in Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	$ 10,508.3
Net Change	108.4	$ (66.1)
Ending balance	10,616.2	10,226.0
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	85.6	(55.0)
Reclassification of Certain Tax Effects from AOCI		(17.8)
Ending balance	(29.3)	(147.6)
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	(4.3)	(3.7)
Reclassification of Certain Tax Effects from AOCI		0.9
Ending balance	(0.3)	1.7
Net Foreign Currency Adjustments
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	67.9	(1.8)
Net Change	14.8	(13.1)
Reclassification of Certain Tax Effects from AOCI		47.5
Ending balance	82.7	32.6
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	12.3	5.7
Reclassification of Certain Tax Effects from AOCI		(55.9)
Ending balance	(398.4)	(392.4)
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(453.7)	(414.3)
Net Change	108.4	(66.1)
Reclassification of Certain Tax Effects from AOCI	0.0	(25.3)
Ending balance	$ (345.3)	$ (505.7)