Asset Quality - Additional Information (Detail)	3 Months Ended		9 Months Ended		12 Months Ended		57 Months Ended
	Sep. 30, 2016 USD ($) SecurityLoan acquisition	Sep. 30, 2015 USD ($) SecurityLoan	Sep. 30, 2016 USD ($) SecurityLoan	Sep. 30, 2015 USD ($) SecurityLoan	Dec. 31, 2015 USD ($)	Dec. 31, 2012 USD ($) acquisition	Sep. 30, 2016 USD ($) SecurityLoan acquisition	Aug. 01, 2016 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2015 USD ($)	Dec. 31, 2014 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
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	80.00%		80.00%				80.00%
Percentage of nonperforming loans outstanding face value	84.00%		84.00%				84.00%
Percentage of loans held for sale and other nonperforming assets	84.00%		84.00%				84.00%
Number of largest nonperforming loans | SecurityLoan	20	20	20	20			20
Aggregate carrying amount of largest nonperforming loans	$ 302,000,000	$ 112,000,000	$ 302,000,000	$ 112,000,000			$ 302,000,000
Percentage of total loans on nonperforming status	42.00%	28.00%	42.00%	28.00%			42.00%
Net reduction to interest income	$ 7,000,000	$ 4,000,000	$ 19,000,000	$ 12,000,000
Total restructured loans	304,000,000	287,000,000	304,000,000	287,000,000	$ 280,000,000		$ 304,000,000
Addition in restructured loans	46,000,000	14,000,000	95,000,000	87,000,000	99,000,000
Restructured loans partially in payments and charge-offs	19,000,000	27,000,000	71,000,000	70,000,000	89,000,000
Financial receivable, modifications, subsequent default, recorded investment	1,000,000	3,000,000	7,000,000	10,000,000
Commitments outstanding to lend additional funds to borrowers	5,000,000	8,000,000	5,000,000	8,000,000	9,000,000		5,000,000
Loans, net of unearned income	85,528,000,000	60,085,000,000	85,528,000,000	60,085,000,000	59,876,000,000		85,528,000,000
Minimum outstanding impaired commercial loans	2,500,000.0		$ 2,500,000.0				2,500,000.0
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	865,000,000	790,000,000	$ 865,000,000	790,000,000	796,000,000		865,000,000
Provision (credit) for loan and lease losses	56,000,000	43,000,000	205,000,000	110,000,000
Liability for credit losses on lending-related commitments	$ 53,000,000	$ 54,000,000	$ 53,000,000	$ 54,000,000	56,000,000		53,000,000		$ 50,000,000	$ 45,000,000	$ 35,000,000
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans | SecurityLoan	0	0	0	0
Consumer Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans | SecurityLoan	23	61	153	215
Continuing Operations [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Increase (decrease) in allowance for loan losses		$ 75,000,000
Percentage of Increase (decrease) in allowance for loan losses		9.50%
Individually Evaluated for Impairment, Outstanding	$ 556,000,000	$ 318,000,000	$ 556,000,000	$ 318,000,000	308,000,000		556,000,000
Individually evaluated for Impairment, corresponding allowance	39,000,000	36,000,000	39,000,000	36,000,000	35,000,000		39,000,000
Collectively Evaluated for Impairment, Outstanding	84,013,000,000	59,755,000,000	84,013,000,000	59,755,000,000	59,557,000,000		84,013,000,000
Collectively evaluated for Impairment, corresponding allowance	826,000,000	753,000,000	826,000,000	753,000,000	760,000,000		826,000,000
Provision (credit) for loan and lease losses	55,000,000	35,000,000	202,000,000	101,000,000
Continuing Operations [Member] | Purchase Credit Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Collectively Evaluated for Impairment, Outstanding	959,000,000	12,000,000	959,000,000	12,000,000	11,000,000		959,000,000		$ 11,000,000	$ 12,000,000	$ 13,000,000
Collectively evaluated for Impairment, corresponding allowance	0	1,000,000	0	1,000,000	1,000,000		0
Provision (credit) for loan and lease losses	0	0	0	0	0
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Interest income recognized on outstanding balances	2,000,000	2,000,000	8,000,000	5,000,000
Loans, net of unearned income	61,384,000,000	44,274,000,000	61,384,000,000	44,274,000,000	44,272,000,000		61,384,000,000
Commercial Portfolio Segment [Member] | Continuing Operations [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Increase (decrease) in allowance for loan losses		$ 71,000,000
Percentage of Increase (decrease) in allowance for loan losses		11.00%
Individually Evaluated for Impairment, Outstanding	337,000,000	$ 92,000,000	337,000,000	92,000,000	83,000,000		337,000,000
Individually evaluated for Impairment, corresponding allowance	16,000,000	10,000,000	16,000,000	10,000,000	8,000,000		16,000,000
Collectively Evaluated for Impairment, Outstanding	60,493,000,000	44,182,000,000	60,493,000,000	44,182,000,000	44,189,000,000		60,493,000,000
Collectively evaluated for Impairment, corresponding allowance	702,000,000	637,000,000	702,000,000	637,000,000	648,000,000		702,000,000
Provision (credit) for loan and lease losses	35,000,000	25,000,000	142,000,000	68,000,000
Consumer Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	24,144,000,000	15,811,000,000	24,144,000,000	15,811,000,000	15,604,000,000		24,144,000,000
Consumer Portfolio Segment [Member] | Continuing Operations [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Increase (decrease) in allowance for loan losses		$ 4,000,000
Percentage of Increase (decrease) in allowance for loan losses		(3.00%)
Individually Evaluated for Impairment, Outstanding	219,000,000	$ 226,000,000	219,000,000	226,000,000	225,000,000		219,000,000
Individually evaluated for Impairment, corresponding allowance	23,000,000	26,000,000	23,000,000	26,000,000	27,000,000		23,000,000
Collectively Evaluated for Impairment, Outstanding	23,520,000,000	15,573,000,000	23,520,000,000	15,573,000,000	15,368,000,000		23,520,000,000
Collectively evaluated for Impairment, corresponding allowance	124,000,000	116,000,000	124,000,000	116,000,000	112,000,000		124,000,000
Provision (credit) for loan and lease losses	20,000,000	10,000,000	60,000,000	33,000,000
Performing [Member] | Loans, Total [Member] | Credit Concentration Risk [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	83,500,000,000	59,300,000,000	$ 83,500,000,000	$ 59,300,000,000	$ 59,200,000,000		83,500,000,000
Concentration risk			97.60%	98.80%	98.90%
Nonperforming [Member] | Loans, Total [Member] | Credit Concentration Risk [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	$ 1,100,000,000	$ 724,000,000	$ 1,100,000,000	$ 724,000,000	$ 667,000,000		$ 1,100,000,000
Concentration risk			1.30%	1.20%	1.10%
Purchase Credit Impaired Loans [Member] | First Niagara Bank, N.A. [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Contractually required payments receivable								$ 23,100,000,000
Fair value of acquired receivables								22,500,000,000
Estimated cash flows not expected to be collected								466,000,000
Accretable yield								$ 29,000,000