Parent Company Financial Statements - Statement of Cash Flows (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Activities:
Net cash provided by operating activities	$ 6,933	$ 2,290	$ (1,403)
Investing Activities:
Net decrease (increase) in interest-bearing deposits with consolidated banking subsidiary	3,708	4,403	18,185
Business acquisitions	0	(437)	0
Net cash provided by investing activities	48	4,230	24,995
Financing Activities:
Proceeds from issuance of long-term debt, net of issuance costs	747	1,492	2,983
Payments for long-term debt	(493)	(1,441)	(1,155)
Proceeds from issuance of preferred stock, net	0	493	742
Proceeds from exercises of common stock options	0	0	4
Purchases of common stock	(1,292)	(1,365)	(1,520)
Repurchases of common stock for employee tax withholding	(126)	(122)	(222)
Payments for cash dividends	(768)	(723)	(655)
Net cash (used in) financing activities	(6,188)	(6,413)	(24,240)
Net increase (decrease)	793	107	(648)
Cash and due from banks at beginning of period	1,314	1,207	1,855
Cash and due from banks at end of period	2,107	1,314	1,207
Parent Company
Operating Activities:
Net cash provided by operating activities	2,047	417	926
Investing Activities:
Net decrease (increase) in interest-bearing deposits with consolidated banking subsidiary	3,103	2,100	295
Investments in consolidated banking and non-banking subsidiaries	(7,672)	(7,600)	(7,959)
Sale or repayment of investment in consolidated banking and non-banking subsidiaries	4,216	6,703	7,891
Business acquisitions	0	(395)	0
Net increase in investments in unconsolidated affiliates	172	0	0
Net cash provided by investing activities	(181)	808	227
Financing Activities:
Net increase (decrease) in commercial paper	0	0	(2,485)
Proceeds from issuance of long-term debt, net of issuance costs	748	1,492	2,983
Payments for long-term debt	(450)	(1,000)	0
Proceeds from issuance of preferred stock, net	0	493	742
Proceeds from exercises of common stock options	0	0	4
Purchases of common stock	(1,292)	(1,365)	(1,520)
Repurchases of common stock for employee tax withholding	(104)	(122)	(222)
Payments for cash dividends	(768)	(723)	(655)
Net cash (used in) financing activities	(1,866)	(1,225)	(1,153)
Net increase (decrease)	0	0	0
Cash and due from banks at beginning of period	0	0	0
Cash and due from banks at end of period	$ 0	$ 0	$ 0