Loans - Summary of Nonaccrual Loans and Accruing Receivables 90 Days or More Delinquent (Detail) - USD ($) $ in Millions		Jun. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total nonaccruing loans		$ 817	$ 1,095
Total accruing loans contractually past due 90 days or more		17	18
Total nonperforming loans		834	1,113
Residential Mortgage Loan Nonaccrual Carried At Lower Of Amortized Cost Or Fair Value Less Cost To Sell		348	360
Commercial Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans		348	613
Total nonaccruing loans		348	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		218	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		117	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		0	0
Consumer Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual Loans		468	481
Total nonaccruing loans		469	482
Total accruing loans contractually past due 90 days or more		16	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]	405	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]	63	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		11	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		$ 5	$ 8

[1] At June 30, 2018 and December 31, 2017, nonaccrual consumer mortgage loans held for investment include $348 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 is 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.