Loans - Summary of TDRs (Detail) - USD ($)		3 Months Ended		6 Months Ended
		Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial loans removed from TDR classification		$ 0	$ 12,000,000	$ 0	$ 12,000,000
TDR Loans	[1]	730,000,000		730,000,000		$ 720,000,000
Residential mortgage loan TDR carried at lower of amortized cost or fair value less cost to sell		501,000,000		501,000,000		557,000,000
Non Accrual loans included in TDR loan		357,000,000		357,000,000		230,000,000
Commercial Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR Loans	[2]	145,000,000		145,000,000		104,000,000
Additional commitments to lend to commercial borrowers with modified TDRs		49,000,000		49,000,000		222,000,000
Commercial Loans [Member] | Business and Corporate Banking [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR Loans		71,000,000		71,000,000		36,000,000
Commercial Loans [Member] | Global Banking [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR Loans		74,000,000		74,000,000		68,000,000
Consumer Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR Loans		585,000,000		585,000,000		616,000,000
Consumer Loans [Member] | Residential Mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR Loans	[3]	549,000,000		549,000,000		580,000,000
Consumer Loans [Member] | Home Equity Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR Loans	[3]	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

[1] At June 30, 2020 and December 31, 2019, the carrying value of TDR Loans includes $357 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 $49 million and $222 million at June 30, 2020 and December 31, 2019, respectively.

[3] At June 30, 2020 and December 31, 2019, the carrying value of consumer mortgage TDR Loans includes $501 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.