Parent Company FInancial Statements Parent Company Financial Statements (Balance Sheets) (Details) - USD ($) $ / shares in Units, $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Condensed Financial Statements, Captions [Line Items]
Cash and due from subsidiary bank	$ 1,438	$ 1,249
Other short-term investments	96	92
Premises and equipment	466	501
Accrued income and other assets	4,495	4,440
Total assets	71,567	72,978
Medium- and long-term debt	4,622	5,160
Other liabilities	1,069	1,012
Total liabilities	63,604	65,182
Common stock - $5 par value: Authorized - 325,000,000 shares; Issued - 228,164,824 shares	1,141	1,141
Capital surplus	2,122	2,135
Accumulated other comprehensive loss	(451)	(383)	$ (429)
Retained earnings	7,887	7,331
Less cost of common stock in treasury - 55,306,483 shares at 12/31/17 and 52,851,156 shares at 12/31/16	(2,736)	(2,428)
Total shareholders' equity	7,963	7,796	$ 7,560	$ 7,402
Total liabilities and shareholders' equity	$ 71,567	$ 72,978
Common stock, par value	$ 5	$ 5
Common stock, authorized shares	325,000,000	325,000,000
Common stock, issued shares	228,164,824	228,164,824
Shares in treasury	55,306,483	52,851,156
Parent Company
Condensed Financial Statements, Captions [Line Items]
Cash and due from subsidiary bank	$ 1,059	$ 761
Other short-term investments	92	87
Investments in subsidiaries, principally banks	7,467	7,561
Premises and equipment	2	2
Accrued income and other assets	127	150
Total assets	8,747	8,561
Medium- and long-term debt	602	604
Other liabilities	182	161
Total liabilities	784	765
Common stock - $5 par value: Authorized - 325,000,000 shares; Issued - 228,164,824 shares	1,141	1,141
Capital surplus	2,122	2,135
Accumulated other comprehensive loss	(451)	(383)
Retained earnings	7,887	7,331
Less cost of common stock in treasury - 55,306,483 shares at 12/31/17 and 52,851,156 shares at 12/31/16	(2,736)	(2,428)
Total shareholders' equity	7,963	7,796
Total liabilities and shareholders' equity	$ 8,747	$ 8,561