Consolidated Statements of Changes in Stockholders' Equity (Unaudited) - USD ($) $ in Millions	Total	Preferred Stock	Common Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Treasury Stock
Beginning balance at Dec. 31, 2014		$ 388.5	$ 408.6	$ 1,050.9	$ 7,625.4	$ (319.7)	$ (704.8)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of Preferred Stock, Series D		0.0
Treasury Stock Transactions — Stock Options and Awards				(69.7)			157.9
Stock Options and Awards — Amortization				59.9
Stock Options and Awards — Tax Benefit				17.6
Net Income	$ 734.5				734.5
Dividends Declared — Common Stock					(248.7)
Dividends Declared — Preferred Stock					(17.5)
Net Unrealized Gains (Losses) on Securities Available for Sale	24.7					24.7
Net Unrealized Gains (Losses) on Cash Flow Hedges	5.3					5.3
Foreign Currency Translation Adjustments	(2.4)					(2.4)
Pension and Other Postretirement Benefit Adjustments	18.4					18.4
Stock Purchased							(344.4)
Ending balance at Sep. 30, 2015	8,784.5	388.5	408.6	1,058.7	8,093.7	(273.7)	(891.3)
Beginning balance at Dec. 31, 2015	8,705.9	388.5	408.6	1,072.3	8,242.8	(372.7)	(1,033.6)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of Preferred Stock, Series D		493.5
Treasury Stock Transactions — Stock Options and Awards				(91.8)			153.9
Stock Options and Awards — Amortization				67.1
Stock Options and Awards — Tax Benefit				(9.0)
Net Income	766.0				766.0
Dividends Declared — Common Stock					(255.1)
Dividends Declared — Preferred Stock					(17.6)
Net Unrealized Gains (Losses) on Securities Available for Sale	94.3					94.3
Net Unrealized Gains (Losses) on Cash Flow Hedges	6.6					6.6
Foreign Currency Translation Adjustments	0.7					0.7
Pension and Other Postretirement Benefit Adjustments	11.4					11.4
Stock Purchased	(346.1)						(346.1)
Ending balance at Sep. 30, 2016	$ 9,579.8	$ 882.0	$ 408.6	$ 1,038.6	$ 8,736.1	$ (259.7)	$ (1,225.8)