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	Series C Preferred Stock PREFERRED STOCK	Series D Preferred Stock PREFERRED STOCK	Parent Company
Balance, beginning of period 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				(74.0)			168.1
Stock Options and Awards – Amortization				77.7
Stock Options and Awards – Tax Benefits	$ 17.7			17.7
Net Income	973.8				973.8					$ 973.8
Dividends Declared – Common Stock					(333.0)
Dividends Declared – Preferred Stock					(23.4)
Net Unrealized (Losses) Gains on Securities Available for Sale	(58.6)					(58.6)
Net Unrealized Gains (Losses) on Cash Flow Hedges	1.7					1.7
Foreign Currency Translation Adjustments	(15.9)					(15.9)
Pension and Other Postretirement Benefit Adjustments	19.8					19.8
Stock Purchased							(496.9)
Balance, end of period at Dec. 31, 2015	8,705.9	388.5	408.6	1,072.3	8,242.8	(372.7)	(1,033.6)	$ 388.5
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of Preferred Stock, Series D									493.5
Treasury Stock Transactions – Stock Options and Awards				(116.6)			350.3
Stock Options and Awards – Amortization				87.7
Stock Options and Awards – Tax Benefits	(7.6)			(7.6)
Net Income	1,032.5				1,032.5					1,032.5
Dividends Declared – Common Stock					(343.5)
Dividends Declared – Preferred Stock					(23.4)
Net Unrealized (Losses) Gains on Securities Available for Sale	(1.4)					(1.4)
Net Unrealized Gains (Losses) on Cash Flow Hedges	9.1					9.1
Foreign Currency Translation Adjustments	(0.9)					(0.9)
Pension and Other Postretirement Benefit Adjustments	(4.1)					(4.1)
Stock Purchased							(411.1)
Balance, end of period at Dec. 31, 2016	9,770.4	882.0	408.6	1,035.8	8,908.4	(370.0)	(1,094.4)			9,770.4
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of Preferred Stock, Series D									$ 0.0
Treasury Stock Transactions – Stock Options and Awards				(117.1)			225.1
Stock Options and Awards – Amortization				128.5
Stock Options and Awards – Tax Benefits	0.0			0.0
Net Income	1,199.0				1,199.0					1,199.0
Dividends Declared – Common Stock					(372.5)
Dividends Declared – Preferred Stock					(49.8)
Net Unrealized (Losses) Gains on Securities Available for Sale	(42.4)					(42.4)
Net Unrealized Gains (Losses) on Cash Flow Hedges	(1.6)					(1.6)
Foreign Currency Translation Adjustments	16.7					16.7
Pension and Other Postretirement Benefit Adjustments	(17.0)					(17.0)
Stock Purchased							(523.1)
Balance, end of period at Dec. 31, 2017	$ 10,216.2	$ 882.0	$ 408.6	$ 1,047.2	$ 9,685.1	$ (414.3)	$ (1,392.4)			$ 10,216.2