Consolidated Statements of Cash Flows - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
OPERATING ACTIVITIES
Net income (loss)	$ 1,298	$ 790	$ 920
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
Provision for credit losses	229	266	166
Depreciation and amortization expense, net	407	314	247
Accretion of acquired loans	203	116	0
Increase in cash surrender value of corporate-owned life insurance	(119)	(111)	(108)
Stock-based compensation expense	100	99	58
FDIC reimbursement (payments), net of FDIC expense	(3)	13	0
Deferred income taxes (benefit)	303	11	(76)
Proceeds from sales of loans held for sale	11,963	8,572	7,333
Originations of loans held for sale, net of repayments	(11,846)	(8,361)	(7,072)
Net losses (gains) from sale of loans held for sale	(181)	(139)	(103)
Net losses (gains) from principal investing	(7)	(20)	(51)
Net losses (gains) and writedown on OREO	5	4	4
Net losses (gains) on leased equipment	3	7	(6)
Net losses (gains) on sales of fixed assets	24	56	8
Net securities losses (gains)	(1)	0	0
Net decrease (increase) in trading account assets	31	(79)	(38)
Direct acquisition costs	0	(44)	0
Other operating activities, net	(594)	195	(151)
NET CASH PROVIDED BY (USED IN) OPERATING ACTIVITIES	1,815	1,689	1,131
INVESTING ACTIVITIES
Cash received (used) in acquisitions, net of cash acquired	(144)	(481)	0
Net decrease (increase) in short-term investments, excluding acquisitions	(1,672)	(68)	1,562
Purchases of securities available for sale	(3,002)	(5,718)	(4,090)
Proceeds from sales of securities available for sale	915	4,249	19
Proceeds from prepayments and maturities of securities available for sale	3,999	4,241	3,098
Proceeds from prepayments and maturities of held-to-maturity securities	1,797	1,627	1,102
Purchases of held-to-maturity securities	(3,398)	(6,968)	(988)
Purchases of other investments	(87)	(46)	(32)
Proceeds from sales of other investments	117	243	145
Proceeds from prepayments and maturities of other investments	4	4	8
Net decrease (increase) in loans, excluding acquisitions, sales and transfers	(945)	(3,580)	(2,951)
Proceeds from sales of portfolio loans	183	140	110
Proceeds from corporate-owned life insurance	55	29	46
Purchases of premises, equipment, and software	(112)	(145)	(75)
Proceeds from sales of premises and equipment	0	0	1
Proceeds from sales of OREO	51	16	22
NET CASH PROVIDED BY (USED IN) INVESTING ACTIVITIES	(2,239)	(6,457)	(2,023)
FINANCING ACTIVITIES
Net increase (decrease) in deposits, excluding acquisitions	1,148	4,047	(952)
Net increase (decrease) in short-term borrowings	(1,299)	(1,294)	(93)
Net proceeds from issuance of long-term debt	2,852	2,827	3,756
Payments on long-term debt	(748)	(1,308)	(1,172)
Issuance of preferred shares	0	1,009	0
Repurchase of Common Shares	(664)	(140)	(448)
Employee equity compensation program Common Share repurchases	(66)	0	0
Redemption of Preferred Stock Series C	(350)	0	0
Net proceeds from reissuance of Common Shares	25	32	22
Cash dividends paid	(480)	(335)	(267)
NET CASH PROVIDED BY (USED IN) FINANCING ACTIVITIES	418	4,838	846
NET INCREASE (DECREASE) IN CASH AND DUE FROM BANKS	(6)	70	(46)
CASH AND DUE FROM BANKS AT BEGINNING OF YEAR	677	607	653
CASH AND DUE FROM BANKS AT END OF YEAR	671	677	607
Additional disclosures relative to cash flows:
Interest paid	598	429	329
Income taxes paid (refunded)	6	144	281
Noncash items:
Reduction of secured borrowing and related collateral	40	67	160
Loans transferred to portfolio from held for sale	105	10	1
Loans transferred to held for sale from portfolio	42	45	63
Loans transferred to other real estate owned	37	36	20
Reinsurance Retention Policy, Excess Retention, Amount Reinsured	18	0	0
First Niagara assets acquired	0	35,616	0
First Niagara liabilities assumed	0	33,028	0
Preferred Stock [Member]
Noncash items:
Common Shares issued for the acquisition of FNFG	0	350	0
Common Stock [Member]
Noncash items:
Common Shares issued for the acquisition of FNFG	$ 0	$ 2,831	$ 0