Loans - Summary of TDRs (Detail) - USD ($)		3 Months Ended		9 Months Ended
		Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Jun. 30, 2021	Dec. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial loans removed from TDR classification		$ 0	$ 0	$ 0	$ 0
TDR Loans	[1]	478,000,000		478,000,000			$ 786,000,000
Residential mortgage loan TDR carried at lower of amortized cost or fair value less cost to sell		339,000,000		339,000,000			487,000,000
Non Accrual loans included in TDR loan		317,000,000		317,000,000			463,000,000
Commercial Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR Loans	[2],[3]	77,000,000		77,000,000			168,000,000
Additional commitments to lend to commercial borrowers with modified TDRs		13,000,000		13,000,000			107,000,000
Commercial Loans [Member] | Cares Act Election [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans excluded from TDR classification		352,000,000		352,000,000			924,000,000
Commercial Loans [Member] | Business and Corporate Banking [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR Loans		40,000,000		40,000,000			94,000,000
Commercial Loans [Member] | Global Banking [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR Loans		37,000,000		37,000,000			74,000,000
Consumer Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR Loans	[4]	401,000,000		401,000,000			618,000,000
Consumer Loans [Member] | Loans Held for Sale [Member] | Exit of Mass Market Retail Banking Business and Remaining Retail Credit Card Portfolio [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR Loans						$ 159,000,000
Consumer Loans [Member] | Residential Mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR Loans	[5],[6]	384,000,000		384,000,000			582,000,000
Consumer Loans [Member] | Home Equity Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR Loans	[5],[6]	17,000,000		17,000,000			31,000,000
Consumer Loans [Member] | Credit Cards [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR Loans		0		0			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		$ 404,000,000		$ 404,000,000			$ 736,000,000

[1] At September 30, 2021 and December 31, 2020, the carrying value of TDR Loans includes $317 million and $463 million, respectively, of loans which are classified as nonaccrua

[2] Additional commitments to lend to commercial borrowers whose loans have been modified in TDR Loans totaled $13 million and $107 million at September 30, 2021 and December 31, 2020, respectively.(2)Not included in the table at September 30, 2021 and December 31, 2020 are $352 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 September 30, 2021 and December 31, 2020 are $352 million and $924 million, respectively, of commercial loans that were exempted from TDR assessment due to our CARES Act election.

[4] The decrease in TDR Loans at September 30, 2021 includes the impact of our decision to exit our mass market retail banking business as well as our remaining retail credit card portfolio which resulted in the transfer of certain loans to held for sale during the second quarter of 2021, including $159 million of consumer TDR Loans.

[5] At September 30, 2021 and December 31, 2020, the carrying value of consumer mortgage TDR Loans includes $339 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.

[6] Not included in the table at September 30, 2021 and December 31, 2020 are $404 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