Loans - Summary of Criticized Assets for Commercial Loans (Detail) - USD ($) $ in Millions		Sep. 30, 2024	Dec. 31, 2023
Financing Receivable, Credit Quality Indicator [Line Items]
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, Total	[1]	$ 62,014	$ 58,987
Commercial loans
Financing Receivable, Credit Quality Indicator [Line Items]
Current year		3,253	4,656
One year prior to current year		4,307	3,868
Two years prior to current year		3,446	3,250
Three years prior to current year		3,128	1,388
Four years prior to current year		1,040	2,354
Prior		12,451	11,069
Revolving Loans		13,179	13,054
Revolving Loans Converted to Term Loans		311	318
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, Total		41,115	39,957
Commercial loans | Real estate, including construction
Financing Receivable, Credit Quality Indicator [Line Items]
Current year		638	920
One year prior to current year		1,039	1,286
Two years prior to current year		1,217	1,019
Three years prior to current year		883	355
Four years prior to current year		262	1,054
Prior		2,715	2,409
Revolving Loans		38	41
Revolving Loans Converted to Term Loans		3	3
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, Total		6,795	7,087
Commercial loans | Business and corporate banking
Financing Receivable, Credit Quality Indicator [Line Items]
Current year		1,409	1,270
One year prior to current year		1,356	1,148
Two years prior to current year		1,118	1,074
Three years prior to current year		1,261	231
Four years prior to current year		176	561
Prior		4,096	4,253
Revolving Loans		6,696	6,922
Revolving Loans Converted to Term Loans		308	265
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, Total		16,420	15,724
Commercial loans | Global banking
Financing Receivable, Credit Quality Indicator [Line Items]
Current year		935	2,255
One year prior to current year		1,768	1,153
Two years prior to current year		849	844
Three years prior to current year		660	151
Four years prior to current year		116	321
Prior		4,495	3,600
Revolving Loans		1,750	2,351
Revolving Loans Converted to Term Loans		0	50
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, Total		10,573	10,725
Commercial loans | Other commercial
Financing Receivable, Credit Quality Indicator [Line Items]
Current year		271	211
One year prior to current year		144	281
Two years prior to current year		262	313
Three years prior to current year		324	651
Four years prior to current year		486	418
Prior		1,145	807
Revolving Loans		4,695	3,740
Revolving Loans Converted to Term Loans		0	0
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, Total		7,327	6,421
Commercial loans | Criticized
Financing Receivable, Credit Quality Indicator [Line Items]
Current year		316	115
One year prior to current year		252	366
Two years prior to current year		316	48
Three years prior to current year		96	136
Four years prior to current year		24	126
Prior		1,436	1,479
Revolving Loans		1,013	1,273
Revolving Loans Converted to Term Loans		17	12
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, Total		3,470	3,555
Commercial loans | Criticized | Real estate, including construction
Financing Receivable, Credit Quality Indicator [Line Items]
Current year		0	2
One year prior to current year		0	40
Two years prior to current year		168	0
Three years prior to current year		33	105
Four years prior to current year		0	92
Prior		788	1,029
Revolving Loans		20	21
Revolving Loans Converted to Term Loans		0	3
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, Total		1,009	1,292
Commercial loans | Criticized | Business and corporate banking
Financing Receivable, Credit Quality Indicator [Line Items]
Current year		316	69
One year prior to current year		252	125
Two years prior to current year		148	48
Three years prior to current year		63	31
Four years prior to current year		4	20
Prior		382	291
Revolving Loans		781	1,113
Revolving Loans Converted to Term Loans		17	9
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, Total		1,963	1,706
Commercial loans | Criticized | Global banking
Financing Receivable, Credit Quality Indicator [Line Items]
Current year		0	0
One year prior to current year		0	201
Two years prior to current year		0	0
Three years prior to current year		0	0
Four years prior to current year		20	14
Prior		266	159
Revolving Loans		212	136
Revolving Loans Converted to Term Loans		0	0
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, Total		498	510
Commercial loans | Criticized | Other commercial
Financing Receivable, Credit Quality Indicator [Line Items]
Current year			44
One year prior to current year			0
Two years prior to current year			0
Three years prior to current year			0
Four years prior to current year			0
Prior			0
Revolving Loans			3
Revolving Loans Converted to Term Loans			0
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, Total			47
Commercial loans | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Current year		172	44
One year prior to current year		128	143
Two years prior to current year		208	12
Three years prior to current year		20	108
Four years prior to current year		4	112
Prior		237	213
Revolving Loans		185	361
Revolving Loans Converted to Term Loans		11	11
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, Total		965	1,004
Commercial loans | Special mention | Real estate, including construction
Financing Receivable, Credit Quality Indicator [Line Items]
Current year		0	0
One year prior to current year		0	40
Two years prior to current year		104	0
Three years prior to current year		0	104
Four years prior to current year		0	92
Prior		18	42
Revolving Loans		0	3
Revolving Loans Converted to Term Loans		0	3
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, Total		122	284
Commercial loans | Special mention | Business and corporate banking
Financing Receivable, Credit Quality Indicator [Line Items]
Current year		172	38
One year prior to current year		128	103
Two years prior to current year		104	12
Three years prior to current year		20	4
Four years prior to current year		4	20
Prior		219	126
Revolving Loans		148	296
Revolving Loans Converted to Term Loans		11	8
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, Total		806	607
Commercial loans | Special mention | Global banking
Financing Receivable, Credit Quality Indicator [Line Items]
Current year		0	0
One year prior to current year		0	0
Two years prior to current year		0	0
Three years prior to current year		0	0
Four years prior to current year		0	0
Prior		0	45
Revolving Loans		37	62
Revolving Loans Converted to Term Loans		0	0
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, Total		37	107
Commercial loans | Special mention | Other commercial
Financing Receivable, Credit Quality Indicator [Line Items]
Current year			6
One year prior to current year			0
Two years prior to current year			0
Three years prior to current year			0
Four years prior to current year			0
Prior			0
Revolving Loans			0
Revolving Loans Converted to Term Loans			0
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, Total			6
Commercial loans | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Current year		138	71
One year prior to current year		118	36
Two years prior to current year		108	0
Three years prior to current year		43	28
Four years prior to current year		20	0
Prior		764	1,091
Revolving Loans		759	819
Revolving Loans Converted to Term Loans		6	1
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, Total		1,956	2,046
Commercial loans | Substandard | Real estate, including construction
Financing Receivable, Credit Quality Indicator [Line Items]
Current year		0	2
One year prior to current year		0	0
Two years prior to current year		64	0
Three years prior to current year		0	1
Four years prior to current year		0	0
Prior		521	821
Revolving Loans		0	0
Revolving Loans Converted to Term Loans		0	0
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, Total		585	824
Commercial loans | Substandard | Business and corporate banking
Financing Receivable, Credit Quality Indicator [Line Items]
Current year		138	31
One year prior to current year		118	22
Two years prior to current year		44	0
Three years prior to current year		43	27
Four years prior to current year		0	0
Prior		157	156
Revolving Loans		593	746
Revolving Loans Converted to Term Loans		6	1
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, Total		1,099	983
Commercial loans | Substandard | Global banking
Financing Receivable, Credit Quality Indicator [Line Items]
Current year		0	0
One year prior to current year		0	14
Two years prior to current year		0	0
Three years prior to current year		0	0
Four years prior to current year		20	0
Prior		86	114
Revolving Loans		166	73
Revolving Loans Converted to Term Loans		0	0
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, Total		272	201
Commercial loans | Substandard | Other commercial
Financing Receivable, Credit Quality Indicator [Line Items]
Current year			38
One year prior to current year			0
Two years prior to current year			0
Three years prior to current year			0
Four years prior to current year			0
Prior			0
Revolving Loans			0
Revolving Loans Converted to Term Loans			0
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, Total			38
Commercial loans | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Current year		6	0
One year prior to current year		6	187
Two years prior to current year		0	36
Three years prior to current year		33	0
Four years prior to current year		0	14
Prior		435	175
Revolving Loans		69	93
Revolving Loans Converted to Term Loans		0	0
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, Total		549	505
Commercial loans | Doubtful | Real estate, including construction
Financing Receivable, Credit Quality Indicator [Line Items]
Current year		0	0
One year prior to current year		0	0
Two years prior to current year		0	0
Three years prior to current year		33	0
Four years prior to current year		0	0
Prior		249	166
Revolving Loans		20	18
Revolving Loans Converted to Term Loans		0	0
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, Total		302	184
Commercial loans | Doubtful | Business and corporate banking
Financing Receivable, Credit Quality Indicator [Line Items]
Current year		6	0
One year prior to current year		6	0
Two years prior to current year		0	36
Three years prior to current year		0	0
Four years prior to current year		0	0
Prior		6	9
Revolving Loans		40	71
Revolving Loans Converted to Term Loans		0	0
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, Total		58	116
Commercial loans | Doubtful | Global banking
Financing Receivable, Credit Quality Indicator [Line Items]
Current year		0	0
One year prior to current year		0	187
Two years prior to current year		0	0
Three years prior to current year		0	0
Four years prior to current year		0	14
Prior		180	0
Revolving Loans		9	1
Revolving Loans Converted to Term Loans		0	0
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, Total		$ 189	202
Commercial loans | Doubtful | Other commercial
Financing Receivable, Credit Quality Indicator [Line Items]
Current year			0
One year prior to current year			0
Two years prior to current year			0
Three years prior to current year			0
Four years prior to current year			0
Prior			0
Revolving Loans			3
Revolving Loans Converted to Term Loans			0
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss, Total			$ 3

[1] The following table summarizes assets and liabilities related to our consolidated variable interest entities ("VIEs") at September 30, 2024 and December 31, 2023. Assets and liabilities exclude intercompany balances that eliminate in consolidation. See Note 15, "Variable Interest Entities," for additional information. September 30, 2024December 31, 2023(in millions)AssetsOther assets$13 $23 Total assets$13 $23 LiabilitiesInterest, taxes and other liabilities$21 $17 Total liabilities$21 $17