Condensed Statement of Cash Flows (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
OPERATING ACTIVITIES:
Net Income	$ 1,032.5	$ 973.8	$ 811.8
Adjustments to Reconcile Net Income to Net Cash Provided by (Used in) Operating Activities:
Change in Accrued Income Taxes	(129.0)	206.8	55.2
Other, net	653.4	(318.1)	86.1
Net Cash Provided by Operating Activities	1,510.0	1,834.4	936.0
INVESTING ACTIVITIES:
Proceeds from Sale, Maturity and Redemption of Securities – Available for Sale	11,317.3	8,576.1	11,476.6
Other, net	226.5	(212.9)	(157.4)
Net Cash Used in Investing Activities	(10,192.1)	(6,900.1)	(8,234.6)
FINANCING ACTIVITIES:
Proceeds from Issuance of Preferred Stock – Series C and Series D	493.5	0.0	388.5
Treasury Stock Purchased	(411.1)	(496.9)	(480.7)
Net Proceeds from Stock Options	233.8	94.0	127.5
Cash Dividends Paid on Common Stock	(333.0)	(321.4)	(302.9)
Cash Dividends Paid on Preferred Stock	(23.4)	(27.0)	0.0
Other, net	(7.5)	17.8	(222.4)
Net Cash Provided by Financing Activities	7,536.9	8,534.1	7,073.7
Increase (Decrease) in Cash and Due from Banks	(1,086.5)	3,397.5	(138.0)
Cash and Due from Banks at Beginning of Year	6,418.5	3,021.0	3,159.0
Cash and Due from Banks at End of Year	5,332.0	6,418.5	3,021.0
Parent Company
OPERATING ACTIVITIES:
Net Income	1,032.5	973.8	811.8
Adjustments to Reconcile Net Income to Net Cash Provided by (Used in) Operating Activities:
Equity in Undistributed Net Income of Subsidiaries	(736.9)	(408.3)	(532.4)
Change in Prepaid Expenses	3.0	1.2	0.2
Change in Accrued Income Taxes	(17.9)	22.8	(30.7)
Other, net	55.7	58.4	56.1
Net Cash Provided by Operating Activities	336.4	647.9	305.0
INVESTING ACTIVITIES:
Proceeds from Sale, Maturity and Redemption of Securities – Available for Sale	0.2	1.3	2.9
Change in Capital Investments in Subsidiaries	(3.0)	(10.0)	0.0
Advances to Wholly-Owned Subsidiaries	(295.0)	0.0	(238.5)
Other, net	1.2	0.2	1.1
Net Cash Used in Investing Activities	(296.6)	(8.5)	(234.5)
FINANCING ACTIVITIES:
Change in Senior Notes and Long-Term Debt	0.0	0.0	(500.0)
Other, net	(0.1)	0.0	0.1
Net Cash Provided by Financing Activities	(40.3)	(751.3)	(767.5)
Increase (Decrease) in Cash and Due from Banks	(0.5)	(111.9)	(697.0)
Cash and Due from Banks at Beginning of Year	757.5	869.4	1,566.4
Cash and Due from Banks at End of Year	$ 757.0	$ 757.5	$ 869.4