Loans - Summary of TDR and Related Credit Loss Reserves for TDR Loans (Detail) - USD ($) $ in Millions		Jun. 30, 2019	Dec. 31, 2018
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[1],[2],[3]	$ 788	$ 859
Unpaid Principal Balance of TDR Loans	[1],[2],[3]	926	1,004
Allowance for credit losses on TDR Loans	[4]	16	17
Commercial Loans [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3],[5]	143	181
Unpaid Principal Balance of TDR Loans	[2],[3],[5]	166	205
Allowance for credit losses on TDR Loans	[4]	12	12
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3]	59	68
Unpaid Principal Balance of TDR Loans	[2],[3]	74	86
Allowance for credit losses on TDR Loans	[4]	12	12
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3]	84	113
Unpaid Principal Balance of TDR Loans	[2],[3]	92	119
Allowance for credit losses on TDR Loans	[4]	0	0
Consumer Loans [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3]	645	678
Unpaid Principal Balance of TDR Loans	[2],[3]	760	799
Allowance for credit losses on TDR Loans	[4]	4	5
Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3],[6]	608	640
Unpaid Principal Balance of TDR Loans	[2],[3],[6]	694	730
Allowance for credit losses on TDR Loans	[4]	3	3
Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3],[6]	33	35
Unpaid Principal Balance of TDR Loans	[2],[3],[6]	62	65
Allowance for credit losses on TDR Loans	[4]	0	1
Consumer Loans [Member] | Credit Cards [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3]	4	3
Unpaid Principal Balance of TDR Loans	[2],[3]	4	4
Allowance for credit losses on TDR Loans	[4]	$ 1	$ 1

[1] At June 30, 2019 and December 31, 2018, the carrying value of TDR Loans includes $256 million and $286 million, respectively, of loans which are classified as nonaccrual.

[2] TDR Loans are considered to be impaired loans. For commercial loans, impaired loans include other loans in addition to TDR Loans which totaled $63 million and $54 million at June 30, 2019 and December 31, 2018, respectively.

[3] The carrying value of TDR Loans includes basis adjustments on the loans, such as partial charge-offs.

[4] Included in the allowance for credit losses.

[5] Additional commitments to lend to commercial borrowers whose loans have been modified in TDR Loans totaled $226 million and $151 million at June 30, 2019 and December 31, 2018, respectively.

[6] At June 30, 2019 and December 31, 2018, the carrying value of consumer mortgage TDR Loans held for investment includes $584 million and $615 million, respectively, of loans that are recorded at the lower of amortized cost or fair value of the collateral less cost to sell.