Loans - Summary of TDRs (Detail) - USD ($)		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial loans removed from TDR classification		$ 0	$ 12,000,000	$ 0
TDR Loans	[1]	786,000,000	720,000,000
Residential mortgage loan TDR carried at lower of amortized cost or fair value less cost to sell		487,000,000	557,000,000
Non Accrual loans included in TDR loan		463,000,000	230,000,000
Commercial Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR Loans	[2],[3]	168,000,000	104,000,000
Additional commitments to lend to commercial borrowers with modified TDRs		107,000,000	222,000,000
Commercial Loans [Member] | Cares Act Election [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans excluded from TDR classification		924,000,000
Commercial Loans [Member] | Business and Corporate Banking [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR Loans		94,000,000	36,000,000
Commercial Loans [Member] | Global Banking [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR Loans		74,000,000	68,000,000
Consumer Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR Loans		618,000,000	616,000,000
Consumer Loans [Member] | Residential Mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR Loans	[4],[5]	582,000,000	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	32,000,000
Consumer Loans [Member] | Credit Cards [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR Loans		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		$ 736,000,000

[1] At December 31, 2020 and 2019, the carrying value of TDR Loans includes $463 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 $107 million and $222 million at December 31, 2020 and 2019, respectively.(2)Not included in the table at December 31, 2020 are $924 million of commercial loans that were exempted from TDR assessment due to our CARES Act election.

[3] Not included in the table at December 31, 2020 are $924 million of commercial loans that were exempted from TDR assessment due to our CARES Act election.

[4] At December 31, 2020 and 2019, the carrying value of consumer mortgage TDR Loans includes $487 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 December 31, 2020 are $736 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 December 31, 2020 are $736 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