Asset Quality - Additional Information (Detail)	3 Months Ended	12 Months Ended				60 Months Ended
	Sep. 30, 2016 acquisition	Dec. 31, 2016 USD ($) SecurityLoan	Dec. 31, 2015 USD ($) SecurityLoan	Dec. 31, 2014 USD ($) SecurityLoan	Dec. 31, 2012 USD ($) acquisition	Dec. 31, 2016 USD ($) acquisition SecurityLoan	Sep. 09, 2016 Branch	Aug. 01, 2016 USD ($)	Dec. 31, 2013 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of branches sold | Branch							18
Number of acquisitions | acquisition	1				1	2
Estimated gross contractual amount receivable of PCI loans					$ 41,000,000
Estimated cash flows nonaccretable amount					11,000,000
Accretable yield					$ 5,000,000
Percentage of carrying amount of our commercial nonperforming loans outstanding		78.00%				78.00%
Percentage of nonperforming loans outstanding face value		81.00%				81.00%
Percentage of loans held for sale and other nonperforming assets		81.00%				81.00%
Number of largest nonperforming loans | SecurityLoan		20	20			20
Aggregate carrying amount of largest nonperforming loans		$ 253,000,000	$ 97,000,000			$ 253,000,000
Percentage of total loans on nonperforming status		41.00%	25.00%			41.00%
Net reduction to interest income		$ 26,000,000	$ 16,000,000	$ 16,000,000
Total restructured loans		280,000,000	280,000,000	270,000,000		$ 280,000,000
Addition in restructured loans		107,000,000	99,000,000	93,000,000
Restructured loans partially in payments and charge-offs		107,000,000	89,000,000	161,000,000
Financial receivable, modifications, subsequent default, recorded investment		9,000,000
Commitments outstanding to lend additional funds to borrowers		14,000,000	9,000,000			14,000,000
Loans net of deferred income		86,038,000,000	59,876,000,000	57,381,000,000		86,038,000,000
Minimum outstanding impaired commercial loans		$ 2,500,000				2,500,000
Number of days to designate commercial loans will be charged off in full or charged down to the fair value of the underlying collateral payment due period		180 days
Number of days to designate the charge-off policy for consumer loans taking effect, payment due period		120 days
Number of days to designate home equity and residential mortgage loans to get charged down to net realizable value payment due period		180 days
Number of days to designate the charge-off policy for credit card loans taking effect, payment due period		180 days
ALLL		$ 858,000,000	796,000,000			858,000,000
Provision (credit) for loan and lease losses		280,000,000	165,000,000	80,000,000
Liability for credit losses on lending-related commitments		55,000,000	$ 56,000,000	$ 35,000,000		55,000,000			$ 37,000,000
Maximum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonperforming loans, collectively evaluated for impairment		$ 2,500,000				2,500,000
Commercial Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans | SecurityLoan		0	2	4
Financial receivable, modifications, subsequent default, recorded investment			$ 1,000,000	$ 11,000,000
Consumer Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans | SecurityLoan		187	269	441
Financial receivable, modifications, subsequent default, recorded investment			$ 12,000,000	$ 15,000,000
Real Estate - Residential Mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financial receivable, modifications, subsequent default, recorded investment		$ 141,000,000	114,000,000
Continuing Operations [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Increase (decrease) in allowance for loan losses		$ 62,000,000
Percentage of Increase (decrease) in allowance for loan losses		7.80%
Individually evaluated for impairment, outstanding		$ 501,000,000	308,000,000			501,000,000
Individually evaluated for Impairment, corresponding allowance		37,000,000	35,000,000			37,000,000
Nonperforming loans, collectively evaluated for impairment		84,672,000,000	59,557,000,000			84,672,000,000
Collectively evaluated for Impairment, corresponding allowance		816,000,000	760,000,000			816,000,000
Provision (credit) for loan and lease losses		267,000,000	144,000,000	59,000,000
Continuing Operations [Member] | Purchase Credit Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonperforming loans, collectively evaluated for impairment		865,000,000	11,000,000	13,000,000		865,000,000
Collectively evaluated for Impairment, corresponding allowance		5,000,000	1,000,000			5,000,000
Provision (credit) for loan and lease losses		11,000,000	0
Commercial Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Interest income recognized on outstanding balances		10,000,000	6,000,000	7,000,000
Commercial Loans [Member] | Commercial Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans net of deferred income		61,909,000,000	44,272,000,000			61,909,000,000
Commercial Loans [Member] | Continuing Operations [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Increase (decrease) in allowance for loan losses		$ 60,000,000
Percentage of Increase (decrease) in allowance for loan losses		9.10%
Commercial Loans [Member] | Continuing Operations [Member] | Commercial Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment, outstanding		$ 289,000,000	83,000,000			289,000,000
Individually evaluated for Impairment, corresponding allowance		17,000,000	8,000,000			17,000,000
Nonperforming loans, collectively evaluated for impairment		61,160,000,000	44,189,000,000			61,160,000,000
Collectively evaluated for Impairment, corresponding allowance		694,000,000	648,000,000			694,000,000
Provision (credit) for loan and lease losses		179,000,000	96,000,000	4,000,000
Consumer Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans net of deferred income			15,604,000,000
Consumer Loans [Member] | Consumer Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans net of deferred income		24,129,000,000				24,129,000,000
Consumer Loans [Member] | Real Estate - Residential Mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans net of deferred income		5,547,000,000	2,242,000,000			5,547,000,000
Consumer Loans [Member] | Continuing Operations [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Increase (decrease) in allowance for loan losses		$ 2,000,000
Percentage of Increase (decrease) in allowance for loan losses		(1.40%)
Consumer Loans [Member] | Continuing Operations [Member] | Consumer Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment, outstanding		$ 212,000,000	225,000,000			212,000,000
Individually evaluated for Impairment, corresponding allowance		20,000,000	27,000,000			20,000,000
Nonperforming loans, collectively evaluated for impairment		23,512,000,000	15,368,000,000			23,512,000,000
Collectively evaluated for Impairment, corresponding allowance		122,000,000	112,000,000			122,000,000
Provision (credit) for loan and lease losses		88,000,000	48,000,000	55,000,000
Consumer Loans [Member] | Continuing Operations [Member] | Real Estate - Residential Mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment, outstanding		51,000,000	56,000,000			51,000,000
Individually evaluated for Impairment, corresponding allowance		2,000,000	4,000,000			2,000,000
Nonperforming loans, collectively evaluated for impairment		5,128,000,000	2,176,000,000			5,128,000,000
Collectively evaluated for Impairment, corresponding allowance		15,000,000	13,000,000			15,000,000
Provision (credit) for loan and lease losses		2,000,000	(2,000,000)	$ (6,000,000)
Performing [Member] | Loans, Total [Member] | Credit Concentration Risk [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans net of deferred income		$ 84,100,000,000	$ 59,200,000,000			84,100,000,000
Concentration risk		97.70%	98.90%
Nonperforming [Member] | Loans, Total [Member] | Credit Concentration Risk [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans net of deferred income		$ 1,100,000,000	$ 667,000,000			$ 1,100,000,000
Concentration risk		1.30%	1.10%
First Niagara Bank, N.A. [Member] | Purchase Credit Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Contractually required payments receivable								$ 22,600,000,000
Fair value of acquired receivables								22,200,000,000
Estimated cash flows not expected to be collected								399,000,000
Accretable yield								$ 205,000,000