Accumulated Other Comprehensive Income (Loss) (Summary of Changes in Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	$ 10,216.2
Net Change	(66.1)	$ 18.0
Ending balance	10,226.0	9,977.7
Net Unrealized Gains (Losses) on Debt Securities Available for Sale
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(74.8)	(32.4)
Net Change	(55.0)	19.3
Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income	(17.8)
Ending balance	(147.6)	(13.1)
Net Unrealized Gains (Losses) on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	4.5	6.1
Net Change	(3.7)	(5.3)
Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income	0.9
Ending balance	1.7	0.8
Net Foreign Currency Adjustments
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(1.8)	(18.5)
Net Change	(13.1)	2.0
Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income	47.5
Ending balance	32.6	(16.5)
Net Pension and Other Postretirement Benefit Adjustments
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(342.2)	(325.2)
Net Change	5.7	2.0
Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income	(55.9)
Ending balance	(392.4)	(323.2)
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(414.3)	(370.0)
Net Change	(66.1)	18.0
Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income	(25.3)	0.0
Ending balance	$ (505.7)	$ (352.0)