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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance			$ 10,216.2
Net Change	$ (2.4)	$ 13.1	(68.5)	$ 31.1
Ending balance	10,357.6	10,067.9	10,357.6	10,067.9
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	(19.9)	1.5	(74.9)	20.9
Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income			(17.8)
Ending balance	(167.5)	(11.5)	(167.5)	(11.5)
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	(3.5)	2.3	(7.2)	(3.0)
Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income			0.9
Ending balance	(1.8)	3.1	(1.8)	3.1
Net Foreign Currency Adjustments
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance			(1.8)	(18.5)
Net Change	13.6	4.9	0.5	6.9
Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income			47.5
Ending balance	46.2	(11.6)	46.2	(11.6)
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	7.4	4.3	13.1	6.3
Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income			(55.9)
Ending balance	(385.0)	(318.9)	(385.0)	(318.9)
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance			(414.3)	(370.0)
Net Change			(68.5)	31.1
Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income			(25.3)	0.0
Ending balance	$ (508.1)	$ (338.9)	$ (508.1)	$ (338.9)