Loans - Summary of TDRs (Detail) - USD ($)		3 Months Ended
		Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial loans removed from TDR classification		$ 0	$ 0
TDR Loans	[1]	783,000,000		$ 786,000,000
Residential mortgage loan TDR carried at lower of amortized cost or fair value less cost to sell		478,000,000		487,000,000
Non Accrual loans included in TDR loan		458,000,000		463,000,000
Commercial Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR Loans	[2],[3]	162,000,000		168,000,000
Additional commitments to lend to commercial borrowers with modified TDRs		35,000,000		107,000,000
Commercial Loans [Member] | Cares Act Election [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans excluded from TDR classification		781,000,000		924,000,000
Commercial Loans [Member] | Business and Corporate Banking [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR Loans		118,000,000		94,000,000
Commercial Loans [Member] | Global Banking [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR Loans		44,000,000		74,000,000
Consumer Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR Loans		621,000,000		618,000,000
Consumer Loans [Member] | Residential Mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR Loans	[4],[5]	585,000,000		582,000,000
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
Consumer Loans [Member] | Credit Cards [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR Loans		5,000,000		5,000,000
Consumer Loans [Member] | Mortgages [Member] | Cares Act Election [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans excluded from TDR classification		$ 646,000,000		$ 736,000,000

[1] At March 31, 2021 and December 31, 2020, the carrying value of TDR Loans includes $458 million and $463 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 $35 million and $107 million at March 31, 2021 and December 31, 2020, respectively.(2)Not included in the table at March 31, 2021 and December 31, 2020 are $781 million and $924 million, respectively, of commercial loans that were exempted from TDR assessment due to our CARES Act election.

[3] Not included in the table at March 31, 2021 and December 31, 2020 are $781 million and $924 million, respectively, of commercial loans that were exempted from TDR assessment due to our CARES Act election.

[4] At March 31, 2021 and December 31, 2020, the carrying value of consumer mortgage TDR Loans includes $478 million and $487 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 March 31, 2021 and December 31, 2020 are $646 million and $736 million, respectively, 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 March 31, 2021 and December 31, 2020 are $646 million and $736 million, respectively, 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