Condensed Balance Sheet (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
ASSETS
Cash on Deposit with Subsidiary Bank	$ 6,444.6	$ 3,050.6	$ 3,162.4	$ 3,752.7
Securities	37,567.4	33,734.0
Other Assets	4,464.4	5,865.1
Total Assets	116,749.6	109,946.5	102,947.3
LIABILITIES
Senior Notes	1,497.4	1,497.0
Total Long-Term Debt	1,371.3	1,615.1
Floating Rate Capital Debt	277.3	277.2
Other Liabilities	3,075.6	3,848.1
Total Liabilities	108,043.7	101,497.6
STOCKHOLDERS’ EQUITY
Preferred Stock	388.5	388.5
Common Stock	408.6	408.6
Retained Earnings	8,242.8	7,625.4
Accumulated Other Comprehensive Loss	(372.7)	(319.7)	(244.3)	(283.0)
Treasury Stock	(1,033.6)	(704.8)
Total Stockholders’ Equity	8,705.9	8,448.9	7,912.0
Total Liabilities and Stockholders’ Equity	116,749.6	109,946.5
Parent Company
ASSETS
Cash on Deposit with Subsidiary Bank	757.5	869.4	$ 1,566.4	$ 6.5
Securities	1.2	2.3
Other Assets	682.3	605.8
Total Assets	11,800.7	11,483.5
LIABILITIES
Senior Notes	1,497.4	1,497.0
Total Long-Term Debt	800.3	793.0
Floating Rate Capital Debt	277.3	277.2
Other Liabilities	519.8	467.4
Total Liabilities	3,094.8	3,034.6
STOCKHOLDERS’ EQUITY
Preferred Stock	388.5	388.5
Common Stock	408.6	408.6
Additional Paid-in Capital	1,072.3	1,050.9
Retained Earnings	8,242.8	7,625.4
Accumulated Other Comprehensive Loss	(372.7)	(319.7)
Treasury Stock	(1,033.6)	(704.8)
Total Stockholders’ Equity	8,705.9	8,448.9
Total Liabilities and Stockholders’ Equity	11,800.7	11,483.5
Parent Company | Banks Industry
ASSETS
Advances to Wholly-Owned Subsidiaries	2,260.0	2,273.5
Investments in Wholly-Owned Subsidiaries	7,927.8	7,604.5
Parent Company | Nonbank Subsidiaries
ASSETS
Advances to Wholly-Owned Subsidiaries	18.5	5.0
Investments in Wholly-Owned Subsidiaries	$ 153.4	$ 123.0