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	Series C Preferred Stock Preferred Stock	Series D Preferred Stock Preferred Stock
Beginning balance at Dec. 31, 2015			$ 408.6	$ 1,072.2	$ 8,242.8	$ (372.7)	$ (1,033.6)	$ 388.5	$ 0.0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Treasury Stock Transactions — Stock Options and Awards				(75.4)			102.2
Stock Options and Awards — Amortization				46.6
Stock Options and Awards — Tax Benefit				(9.1)
Net Income	$ 508.4				508.4
Dividends Declared — Common Stock					(167.3)
Dividends Declared — Preferred Stock					(11.7)
Net Unrealized Gains on Securities Available for Sale	108.2					108.2
Net Unrealized Gains (Losses) on Cash Flow Hedges	6.6					6.6
Foreign Currency Translation Adjustments	10.2					10.2
Pension and Other Postretirement Benefit Adjustments	8.1					8.1
Stock Purchased							(281.1)
Ending balance at Jun. 30, 2016	8,951.5	$ 388.5	408.6	1,034.3	8,572.2	(239.6)	(1,212.5)
Beginning balance at Dec. 31, 2016	9,770.4		408.6	1,035.8	8,908.4	(370.0)	(1,094.4)	$ 388.5	$ 493.5
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Treasury Stock Transactions — Stock Options and Awards				(93.6)			163.8
Stock Options and Awards — Amortization				84.8
Stock Options and Awards — Tax Benefit				0.0
Net Income	544.0				544.0
Dividends Declared — Common Stock					(178.3)
Dividends Declared — Preferred Stock					(26.6)
Net Unrealized Gains on Securities Available for Sale	20.9					20.9
Net Unrealized Gains (Losses) on Cash Flow Hedges	(3.0)					(3.0)
Foreign Currency Translation Adjustments	6.9					6.9
Pension and Other Postretirement Benefit Adjustments	6.3					6.3
Stock Purchased	(227.7)						(227.7)
Ending balance at Jun. 30, 2017	$ 10,067.9	$ 882.0	$ 408.6	$ 1,027.0	$ 9,247.5	$ (338.9)	$ (1,158.3)