Consolidated Statements of Cash Flows - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
OPERATING ACTIVITIES
Net income (loss)	$ 1,866	$ 1,298	$ 790
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
Provision for credit losses	246	229	266
Depreciation and amortization expense, net	382	407	314
Accretion of acquired loans	86	203	116
Increase in cash surrender value of corporate-owned life insurance	(117)	(119)	(111)
Stock-based compensation expense	99	100	99
FDIC reimbursement (payments), net of FDIC expense	(10)	(3)	13
Deferred income taxes (benefit)	98	303	11
Proceeds from sales of loans held for sale	14,019	11,963	8,572
Originations of loans held for sale, net of repayments	(13,948)	(11,846)	(8,361)
Net losses (gains) from sale of loans held for sale	(183)	(181)	(139)
Net losses (gains) and writedown on OREO	0	5	4
Net losses (gains) on leased equipment	41	3	7
Net losses (gains) on sales of fixed assets	9	24	56
Net securities losses (gains)	0	(1)	0
Net decrease (increase) in trading account assets	(13)	31	(79)
Gain on sale of KIBS	(83)	0	0
Direct acquisition costs	0	0	(44)
Other operating activities, net	14	(601)	175
NET CASH PROVIDED BY (USED IN) OPERATING ACTIVITIES	2,506	1,815	1,689
INVESTING ACTIVITIES
Cash received (used) in acquisitions, net of cash acquired	0	(144)	(481)
Proceeds from sale of KIBS	124	0	0
Net decrease (increase) in short-term investments, excluding acquisitions	1,885	(1,672)	(68)
Purchases of securities available for sale	(4,594)	(3,002)	(5,718)
Proceeds from sales of securities available for sale	0	915	4,249
Proceeds from prepayments and maturities of securities available for sale	3,197	3,999	4,241
Proceeds from prepayments and maturities of held-to-maturity securities	1,558	1,797	1,627
Purchases of held-to-maturity securities	(1,242)	(3,398)	(6,968)
Purchases of other investments	(28)	(87)	(46)
Proceeds from sales of other investments	62	117	243
Proceeds from prepayments and maturities of other investments	40	4	4
Net decrease (increase) in loans, excluding acquisitions, sales, and transfers	(3,700)	(945)	(3,580)
Proceeds from sales of portfolio loans	204	183	140
Proceeds from corporate-owned life insurance	78	55	29
Purchases of premises, equipment, and software	(99)	(112)	(145)
Proceeds from sales of premises and equipment	2	0	0
Proceeds from sales of OREO	31	51	16
NET CASH PROVIDED BY (USED IN) INVESTING ACTIVITIES	(2,482)	(2,239)	(6,457)
FINANCING ACTIVITIES
Net increase (decrease) in deposits, excluding acquisitions	2,074	1,148	4,047
Net increase (decrease) in short-term borrowings	(148)	(1,299)	(1,294)
Net proceeds from issuance of long-term debt	2,306	2,852	2,827
Payments on long-term debt	(2,880)	(748)	(1,308)
Issuance of preferred shares	412	0	1,009
Repurchase of Common Shares	(1,098)	(664)	(140)
Employee equity compensation program Common Share repurchases	(47)	(66)	0
Redemption of Preferred Stock Series C	0	(350)	0
Net proceeds from reissuance of Common Shares	20	25	32
Cash dividends paid	(656)	(480)	(335)
NET CASH PROVIDED BY (USED IN) FINANCING ACTIVITIES	(17)	418	4,838
NET INCREASE (DECREASE) IN CASH AND DUE FROM BANKS	7	(6)	70
CASH AND DUE FROM BANKS AT BEGINNING OF YEAR	671	677	607
CASH AND DUE FROM BANKS AT END OF YEAR	678	671	677
Additional disclosures relative to cash flows:
Interest paid	892	598	429
Income taxes paid (refunded)	12	6	144
Noncash items:
Reduction of secured borrowing and related collateral	20	40	67
Loans transferred to portfolio from held for sale	24	105	10
Loans transferred to held for sale from portfolio	(33)	42	45
Loans transferred to other real estate owned	25	37	36
CMBS risk retentions	16	18	0
First Niagara assets acquired	0	0	35,616
First Niagara liabilities assumed	0	0	33,028
Preferred Stock [Member]
Noncash items:
Common Shares issued for the acquisition of FNFG	0	0	350
Common Stock [Member]
Noncash items:
Common Shares issued for the acquisition of FNFG	$ 0	$ 0	$ 2,831