Consolidated Statements of Changes in Stockholders' Equity - 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, 2017		$ 882.0	$ 408.6	$ 1,047.3	$ 9,685.1	$ (414.3)	$ (1,392.4)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock Options and Awards				(86.7)			108.6
Stock Options and Awards — Amortization				89.6
Reclassification of Certain Tax Effects from AOCI					25.3	(25.3)
Net Income	$ 772.0				772.0
Dividends Declared — Common Stock					(192.2)
Dividends Declared — Preferred Stock					(23.2)
Net Unrealized Gains (Losses) on Debt Securities Available for Sale	(74.9)					(74.9)
Net Unrealized Gains (Losses) on Cash Flow Hedges	(7.2)					(7.2)
Foreign Currency Translation Adjustments	0.5					0.5
Pension and Other Postretirement Benefit Adjustments	13.1					13.1
Stock Purchased							(453.8)
Ending balance at Jun. 30, 2018	10,357.6	882.0	408.6	1,050.2	10,262.5	(508.1)	(1,737.6)
Beginning balance at Dec. 31, 2018	10,508.3	882.0	408.6	1,068.5	10,776.8	(453.7)	(2,173.9)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock Options and Awards				(128.8)			137.9
Stock Options and Awards — Amortization				74.1
Reclassification of Certain Tax Effects from AOCI					0.0	0.0
Net Income	736.5				736.5
Dividends Declared — Common Stock					(264.6)
Dividends Declared — Preferred Stock					(23.2)
Net Unrealized Gains (Losses) on Debt Securities Available for Sale	248.0					248.0
Net Unrealized Gains (Losses) on Cash Flow Hedges	0.5					0.5
Foreign Currency Translation Adjustments	28.9					28.9
Pension and Other Postretirement Benefit Adjustments	16.5					16.5
Stock Purchased	(528.6)						(528.6)
Ending balance at Jun. 30, 2019	$ 10,805.5	$ 882.0	$ 408.6	$ 1,013.8	$ 11,225.5	$ (159.8)	$ (2,564.6)