Parent Company Financial Statements - Parent Company Condensed Statements of Cash Flows (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash Flows From Operating Activities:
Net Income	$ 2,415	$ 2,442	$ 2,123
Net change in operating assets and liabilities:
Other, net	(120)	(62)	128
Net cash from operating activities	4,635	3,115	3,131
Cash Flows From Investing Activities:
Proceeds from maturities, calls and paydowns of AFS securities	4,800	5,888	5,064
Purchases of AFS securities	(7,397)	(10,033)	(12,698)
Proceeds from maturities, calls and paydowns of HTM securities	2,580	7,022	3,791
Net cash from acquisitions and divestitures	0	(785)	1,055
Other, net	(31)	398	390
Net cash from investing activities	(5,309)	(1,008)	(1,849)
Cash Flows From Financing Activities:
Net change in short-term borrowings	3,532	(3,581)	(982)
Repurchase of common stock	(1,613)	(520)	0
Net cash from common stock transactions in connection with equity awards	108	218	68
Net proceeds from preferred stock issued	0	450	0
Other, net	15	107	(390)
Net cash from financing activities	(549)	(1,882)	104
Net Change in Cash and Cash Equivalents	(1,223)	225	1,386
Cash and Cash Equivalents at Beginning of Period	3,936	3,711	2,325
Cash and Cash Equivalents at End of Period	2,713	3,936	3,711
Parent Company
Cash Flows From Operating Activities:
Net Income	2,415	2,442	2,123
Adjustments to reconcile net income to net cash from operating activities:
Equity in earnings of subsidiaries in excess of dividends from subsidiaries	(558)	(1,188)	(273)
Net change in operating assets and liabilities:
Other, net	0	(14)	35
Net cash from operating activities	1,857	1,240	1,885
Cash Flows From Investing Activities:
Proceeds from maturities, calls and paydowns of AFS securities	29	27	49
Purchases of AFS securities	(29)	(31)	(21)
Proceeds from maturities, calls and paydowns of HTM securities	0	2	27
Investment in subsidiaries	1,100	(85)	17
Advances to subsidiaries	(6,958)	(7,719)	(7,461)
Proceeds from repayment of advances to subsidiaries	4,671	6,975	6,831
Net cash from acquisitions and divestitures	0	(254)	(595)
Other, net	1	0	0
Net cash from investing activities	(1,186)	(1,085)	(1,153)
Cash Flows From Financing Activities:
Net change in short-term borrowings	(39)	(60)	30
Net change in long-term debt	1,319	465	(92)
Repurchase of common stock	(1,613)	(520)	0
Net cash from common stock transactions in connection with equity awards	108	218	68
Net proceeds from preferred stock issued	0	450	0
Cash dividends paid on common and preferred stock	(1,179)	(1,092)	(937)
Other, net	(4)	7	0
Net cash from financing activities	(1,408)	(532)	(931)
Net Change in Cash and Cash Equivalents	(737)	(377)	(199)
Cash and Cash Equivalents at Beginning of Period	7,115	7,492	7,691
Cash and Cash Equivalents at End of Period	$ 6,378	$ 7,115	$ 7,492