Loans - Summary of Nonaccrual Loans and Accruing Receivables 90 Days or More Delinquent (Detail) - USD ($) $ in Millions		Mar. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total nonaccruing loans		$ 1,056	$ 1,095
Total accruing loans contractually past due 90 days or more		15	18
Total nonperforming loans		1,071	1,113
Residential Mortgage Loan Nonaccrual Carried At Lower Of Amortized Cost Or Fair Value Less Cost To Sell		362	360
Commercial Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans		550	613
Total nonaccruing loans		581	613
Total accruing loans contractually past due 90 days or more		1	1
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans		13	12
Total accruing loans contractually past due 90 days or more		0	0
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans		258	215
Total accruing loans contractually past due 90 days or more		1	1
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans		279	385
Total accruing loans contractually past due 90 days or more		0	0
Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans		0	1
Total accruing loans contractually past due 90 days or more		0	0
Commercial Loans [Member] | Nonaccrual Loans Held-for-sale [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total nonaccruing loans		31	0
Consumer Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans		474	481
Total nonaccruing loans		475	482
Total accruing loans contractually past due 90 days or more		14	17
Consumer Loans [Member] | Nonaccrual Loans Held-for-sale [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total nonaccruing loans		1	1
Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans	[1],[2],[3]	408	414
Total accruing loans contractually past due 90 days or more		0	0
Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans	[1],[3]	66	67
Total accruing loans contractually past due 90 days or more		0	0
Consumer Loans [Member] | Credit Cards [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans		0	0
Total accruing loans contractually past due 90 days or more		8	9
Consumer Loans [Member] | Other consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans		0	0
Total accruing loans contractually past due 90 days or more		$ 6	$ 8

[1] At March 31, 2018 and December 31, 2017, nonaccrual consumer mortgage loans held for investment include $362 million and $360 million, respectively, of loans that are carried at the lower of amortized cost or fair value of the collateral less cost to sell.

[2] Nonaccrual consumer mortgage loans for all periods does not include guaranteed loans purchased from the Government National Mortgage Association. Repayment of these loans are predominantly insured by the Federal Housing Administration and as such, these loans have different risk characteristics from the rest of our consumer loan portfolio.

[3] Nonaccrual consumer mortgage loans held for investment include all loans which are 90 or more days contractually delinquent as well as loans discharged under Chapter 7 bankruptcy and not re-affirmed and second lien loans where the first lien loan that we own or service is 90 or more days contractually delinquent.