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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Change in Accounting Principle					$ 0.0
Beginning balance at Dec. 31, 2016		$ 882.0	$ 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				(81.8)			119.0
Stock Options and Awards — Amortization				56.2
Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income					0.0	0.0
Net Income	$ 276.1				276.1
Dividends Declared — Common Stock					(89.4)
Dividends Declared — Preferred Stock					(20.7)
Net Unrealized (Losses) Gains on Debt Securities Available for Sale	19.3					19.3
Net Unrealized (Losses) Gains on Cash Flow Hedges	(5.3)					(5.3)
Foreign Currency Translation Adjustments	2.0					2.0
Pension and Other Postretirement Benefit Adjustments	2.0					2.0
Stock Purchased							(70.1)
Ending balance at Mar. 31, 2017	9,977.7			1,010.2	9,074.4	(352.0)	(1,045.5)	388.5	493.5
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Change in Accounting Principle					(4.5)
Beginning balance at Dec. 31, 2017	10,216.2	$ 882.0	408.6	1,047.3	9,685.1	(414.3)	(1,392.4)		493.5
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Treasury Stock Transactions — Stock Options and Awards				(83.5)			102.5
Stock Options and Awards — Amortization				56.6
Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income					25.3	(25.3)
Net Income	381.6				381.6
Dividends Declared — Common Stock					(96.4)
Dividends Declared — Preferred Stock					(17.3)
Net Unrealized (Losses) Gains on Debt Securities Available for Sale	(55.0)					(55.0)
Net Unrealized (Losses) Gains on Cash Flow Hedges	(3.7)					(3.7)
Foreign Currency Translation Adjustments	(13.1)					(13.1)
Pension and Other Postretirement Benefit Adjustments	5.7					5.7
Stock Purchased	(263.2)						(263.2)
Ending balance at Mar. 31, 2018	$ 10,226.0		$ 408.6	$ 1,020.4	$ 9,973.8	$ (505.7)	$ (1,553.1)	$ 388.5	$ 493.5