Loans - Summary of TDRs (Detail) - USD ($)		3 Months Ended		9 Months Ended
		Sep. 30, 2020	Jun. 30, 2019	Sep. 30, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial loans removed from TDR classification		$ 0	$ 12,000,000	$ 0
TDR Loans	[1]	744,000,000		744,000,000	$ 720,000,000
Residential mortgage loan TDR carried at lower of amortized cost or fair value less cost to sell		490,000,000		490,000,000	557,000,000
Non Accrual loans included in TDR loan		359,000,000		359,000,000	230,000,000
Commercial Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR Loans	[2],[3]	155,000,000		155,000,000	104,000,000
Additional commitments to lend to commercial borrowers with modified TDRs		99,000,000		99,000,000	222,000,000
Commercial Loans [Member] | Cares Act Election [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans excluded from TDR classification		1,103,000,000		1,103,000,000
Commercial Loans [Member] | Business and Corporate Banking [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR Loans		120,000,000		120,000,000	36,000,000
Commercial Loans [Member] | Global Banking [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR Loans		35,000,000		35,000,000	68,000,000
Consumer Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR Loans		589,000,000		589,000,000	616,000,000
Consumer Loans [Member] | Residential Mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR Loans	[4],[5]	553,000,000		553,000,000	580,000,000
Consumer Loans [Member] | Home Equity Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR Loans	[4],[5]	31,000,000		31,000,000	32,000,000
Consumer Loans [Member] | Credit Cards [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR Loans		5,000,000		5,000,000	$ 4,000,000
Consumer Loans [Member] | Mortgages [Member] | Cares Act Election [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans excluded from TDR classification		$ 24,000,000		$ 24,000,000

[1] At September 30, 2020 and December 31, 2019, the carrying value of TDR Loans includes $359 million and $230 million, respectively, of loans which are classified as nonaccrual.

[2] Additional commitments to lend to commercial borrowers whose loans have been modified in TDR Loans totaled $99 million and $222 million at September 30, 2020 and December 31, 2019, respectively.⁽²⁾Not included in the table at September 30, 2020 are $1,103 million of commercial loans that were exempted from TDR assessment due to our CARES Act election.

[3] Not included in the table at September 30, 2020 are $1,103 million of commercial loans that were exempted from TDR assessment due to our CARES Act election.

[4] At September 30, 2020 and December 31, 2019, the carrying value of consumer mortgage TDR Loans includes $490 million and $557 million, respectively, of loans that are recorded at the lower of amortized cost or fair value of the collateral less cost to sell.⁽⁴⁾Not included in the table at September 30, 2020 are $24 million of consumer mortgage loans under COVID-19 related payment deferral programs where the borrowers were provided with extended payment deferral relief of more than six months that were exempted from TDR assessment due to our CARES Act election.

[5] Not included in the table at September 30, 2020 are $24 million of consumer mortgage loans under COVID-19 related payment deferral programs where the borrowers were provided with extended payment deferral relief of more than six months that were exempted from TDR assessment due to our CARES Act election