Consolidated Statements of Cash Flows - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
OPERATING ACTIVITIES
Net income (loss)	$ 1,101	$ 562
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
Provision for credit losses	180	200
Depreciation and amortization expense, net	301	293
Accretion of acquired loans	171	0
Increase in cash surrender value of corporate-owned life insurance	(85)	(76)
Stock-based compensation expense	74	66
FDIC reimbursement (payments), net of FDIC expense	(2)	7
Deferred income taxes (benefit)	134	(63)
Proceeds from sales of loans held for sale	8,109	5,181
Originations of loans held for sale, net of repayments	(8,287)	(5,516)
Net losses (gains) on sales of loans held for sale	(111)	(92)
Net losses (gains) from principal investing	(4)	(16)
Net losses (gains) and writedown on OREO	3	3
Net losses (gains) on leased equipment	2	10
Net securities losses (gains)	(1)	6
Net losses (gains) on sales of fixed assets	16	13
Net decrease (increase) in trading account assets	84	(138)
Other operating activities, net	(606)	420
NET CASH PROVIDED BY (USED IN) OPERATING ACTIVITIES	1,079	860
INVESTING ACTIVITIES
Cash received (used) in acquisitions, net of cash acquired	(74)	(481)
Net decrease (increase) in short-term investments, excluding acquisitions	(1,218)	(509)
Purchases of securities available for sale	(2,725)	(4,203)
Proceeds from sales of securities available for sale	914	4,248
Proceeds from prepayments and maturities of securities available for sale	3,038	2,867
Proceeds from prepayments and maturities of held-to-maturity securities	1,350	1,048
Purchases of held-to-maturity securities	(1,395)	(5,150)
Purchases of other investments	(78)	(28)
Proceeds from sales of other investments	99	204
Proceeds from prepayments and maturities of other investments	2	3
Net decrease (increase) in loans, excluding acquisitions, sales and transfers	(896)	(2,501)
Proceeds from sales of portfolio loans	129	100
Proceeds from corporate-owned life insurance	40	24
Purchases of premises, equipment, and software	(56)	(79)
Proceeds from sales of premises and equipment	0	0
Proceeds from sales of OREO	37	13
NET CASH PROVIDED BY (USED IN) INVESTING ACTIVITIES	(833)	(4,444)
FINANCING ACTIVITIES
Net increase (decrease) in deposits, excluding acquisitions	(641)	4,147
Net increase (decrease) in short-term borrowings	(1,322)	(2,193)
Net proceeds from issuance of long-term debt	2,850	2,078
Payments on long-term debt	(43)	(533)
Issuance of preferred shares	0	519
Open market common share repurchases	(466)	(73)
Employee equity compensation program common share repurchases	(65)	0
Redemption of Preferred Stock Series C	(350)	0
Net proceeds from reissuance of common shares	29	5
Cash dividends paid	(353)	(224)
NET CASH PROVIDED BY (USED IN) FINANCING ACTIVITIES	(361)	3,726
NET INCREASE (DECREASE) IN CASH AND DUE FROM BANKS	(115)	142
CASH AND DUE FROM BANKS AT BEGINNING OF PERIOD	677	607
CASH AND DUE FROM BANKS AT END OF PERIOD	562	749
Additional disclosures relative to cash flows:
Interest paid	442	308
Income taxes paid (refunded)	3	68
Noncash items:
Reduction of secured borrowing and related collateral	33	59
Loans transferred to portfolio from held for sale	93	8
Loans transferred to held for sale from portfolio	41	32
Loans transferred to OREO	29	15
Common Stock [Member]
Noncash items:
Stock issued to acquire First Niagara	0	2,831
Preferred Stock [Member]
Noncash items:
Stock issued to acquire First Niagara	$ 0	$ 350