Loans - Summary of Nonaccrual Loans and Accruing Receivables 90 Days or More Delinquent (Detail) - USD ($) $ in Millions		Sep. 30, 2019	Dec. 31, 2018
Financing Receivable, Past Due [Line Items]
Total nonaccruing loans		$ 683	$ 540
Total accruing loans contractually past due 90 days or more		25	21
Total nonperforming loans		708	561
Residential Mortgage Loan Nonaccrual Carried At Lower Of Amortized Cost Or Fair Value Less Cost To Sell		273	289
Commercial Loans [Member]
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans		264	143
Total nonaccruing loans		264	143
Total accruing loans contractually past due 90 days or more		1	1
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans		3	7
Total accruing loans contractually past due 90 days or more		0	0
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans		149	70
Total accruing loans contractually past due 90 days or more		1	1
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans		112	65
Total accruing loans contractually past due 90 days or more		0	0
Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans		0	1
Total accruing loans contractually past due 90 days or more		0	0
Consumer Loans [Member]
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans		418	396
Total nonaccruing loans		419	397
Total accruing loans contractually past due 90 days or more		24	20
Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	[1],[2],[3]	371	341
Total accruing loans contractually past due 90 days or more		0	0
Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans	[1],[3]	47	55
Total accruing loans contractually past due 90 days or more		0	0
Consumer Loans [Member] | Nonaccrual Loans Held-for-sale [Member]
Financing Receivable, Past Due [Line Items]
Total nonaccruing loans		1	1
Consumer Loans [Member] | Credit Cards [Member]
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans		0	0
Total accruing loans contractually past due 90 days or more		18	14
Consumer Loans [Member] | Other consumer [Member]
Financing Receivable, Past Due [Line Items]
Nonaccrual Loans		0	0
Total accruing loans contractually past due 90 days or more		$ 6	$ 6

[1] At September 30, 2019 and December 31, 2018, nonaccrual consumer mortgage loans held for investment include $273 million and $289 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.