Accumulated Other Comprehensive Income (Loss) (Summary of Changes in Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance			$ 9,770.4
Net Change	$ 3.8	$ (20.1)	34.9	$ 113.0
Ending balance	10,165.2	9,579.8	10,165.2	9,579.8
Net Unrealized Gains (Losses) on Securities Available for Sale
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance			(32.4)	(31.0)
Net Change	(2.2)	(13.9)	18.7	94.3
Ending balance	(13.7)	63.3	(13.7)	63.3
Net Unrealized Gains (Losses) on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance			6.1	(3.0)
Net Change	(1.9)	0.1	(4.9)	6.6
Ending balance	1.2	3.6	1.2	3.6
Net Foreign Currency Adjustments
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance			(18.5)	(17.6)
Net Change	3.6	(9.5)	10.5	0.7
Ending balance	(8.0)	(16.9)	(8.0)	(16.9)
Net Pension and Other Postretirement Benefit Adjustments
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance			(325.2)	(321.1)
Net Change	4.3	3.3	10.6	11.4
Ending balance	(314.6)	(309.7)	(314.6)	(309.7)
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance			(370.0)	(372.7)
Net Change			34.9	113.0
Ending balance	$ (335.1)	$ (259.7)	$ (335.1)	$ (259.7)