Loans - Summary of Criticized Assets for Commercial Loans (Detail) - USD ($) $ in Millions		Mar. 31, 2021	Dec. 31, 2020
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans		$ 61,526	$ 62,088
Commercial Loans [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		207	2,510
Fiscal year before current fiscal year		2,808	5,322
Two years before current fiscal year		5,060	3,469
Three years before current fiscal year		3,446	2,039
Four years before current fiscal year		1,969	639
Prior		12,877	11,882
Revolving Loans		14,461	15,396
Revolving Loans Converted to Term Loans		324	342
Total loans		41,152	41,599
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		0	545
Fiscal year before current fiscal year		563	3,775
Two years before current fiscal year		3,663	2,799
Three years before current fiscal year		2,785	1,368
Four years before current fiscal year		1,300	264
Prior		1,703	1,614
Revolving Loans		56	79
Revolving Loans Converted to Term Loans		17	20
Total loans		10,087	10,464
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		46	1,080
Fiscal year before current fiscal year		1,204	621
Two years before current fiscal year		637	265
Three years before current fiscal year		247	335
Four years before current fiscal year		318	152
Prior		4,076	3,488
Revolving Loans		6,290	7,216
Revolving Loans Converted to Term Loans		307	322
Total loans	[1]	13,125	13,479
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		117	507
Fiscal year before current fiscal year		605	495
Two years before current fiscal year		358	190
Three years before current fiscal year		199	231
Four years before current fiscal year		238	104
Prior		6,318	6,150
Revolving Loans		5,322	5,842
Revolving Loans Converted to Term Loans		0	0
Total loans	[2]	13,157	13,519
Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		44	378
Fiscal year before current fiscal year		436	431
Two years before current fiscal year		402	215
Three years before current fiscal year		215	105
Four years before current fiscal year		113	119
Prior		780	630
Revolving Loans		2,793	2,259
Revolving Loans Converted to Term Loans		0	0
Total loans		4,783	4,137
Commercial Loans [Member] | Criticized [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		0	18
Fiscal year before current fiscal year		23	607
Two years before current fiscal year		631	231
Three years before current fiscal year		300	395
Four years before current fiscal year		446	126
Prior		1,748	1,604
Revolving Loans		1,517	1,867
Revolving Loans Converted to Term Loans		41	49
Total loans		4,706	4,897
Commercial Loans [Member] | Criticized [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		0	0
Fiscal year before current fiscal year		0	492
Two years before current fiscal year		577	115
Three years before current fiscal year		205	340
Four years before current fiscal year		378	85
Prior		560	523
Revolving Loans		0	0
Revolving Loans Converted to Term Loans		0	0
Total loans		1,720	1,555
Commercial Loans [Member] | Criticized [Member] | Business and Corporate Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		0	18
Fiscal year before current fiscal year		23	115
Two years before current fiscal year		54	68
Three years before current fiscal year		95	55
Four years before current fiscal year		60	41
Prior		734	612
Revolving Loans		1,019	1,092
Revolving Loans Converted to Term Loans		41	49
Total loans		2,026	2,050
Commercial Loans [Member] | Criticized [Member] | Global Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		0	0
Fiscal year before current fiscal year		0	0
Two years before current fiscal year		0	48
Three years before current fiscal year		0	0
Four years before current fiscal year		0	0
Prior		334	355
Revolving Loans		458	735
Revolving Loans Converted to Term Loans		0	0
Total loans		792	1,138
Commercial Loans [Member] | Criticized [Member] | Other Commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		0	0
Fiscal year before current fiscal year		0	0
Two years before current fiscal year		0	0
Three years before current fiscal year		0	0
Four years before current fiscal year		8	0
Prior		120	114
Revolving Loans		40	40
Revolving Loans Converted to Term Loans		0	0
Total loans		168	154
Commercial Loans [Member] | Special Mention [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		0	17
Fiscal year before current fiscal year		18	377
Two years before current fiscal year		183	158
Three years before current fiscal year		268	203
Four years before current fiscal year		247	95
Prior		994	1,013
Revolving Loans		685	738
Revolving Loans Converted to Term Loans		36	48
Total loans		2,431	2,649
Commercial Loans [Member] | Special Mention [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		0	0
Fiscal year before current fiscal year		0	306
Two years before current fiscal year		162	115
Three years before current fiscal year		205	171
Four years before current fiscal year		210	85
Prior		376	437
Revolving Loans		0	0
Revolving Loans Converted to Term Loans		0	0
Total loans		953	1,114
Commercial Loans [Member] | Special Mention [Member] | Business and Corporate Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		0	17
Fiscal year before current fiscal year		18	71
Two years before current fiscal year		21	43
Three years before current fiscal year		63	32
Four years before current fiscal year		29	10
Prior		463	390
Revolving Loans		575	600
Revolving Loans Converted to Term Loans		36	48
Total loans		1,205	1,211
Commercial Loans [Member] | Special Mention [Member] | Global Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		0	0
Fiscal year before current fiscal year		0	0
Two years before current fiscal year		0	0
Three years before current fiscal year		0	0
Four years before current fiscal year		0	0
Prior		105	142
Revolving Loans		70	98
Revolving Loans Converted to Term Loans		0	0
Total loans		175	240
Commercial Loans [Member] | Special Mention [Member] | Other Commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		0	0
Fiscal year before current fiscal year		0	0
Two years before current fiscal year		0	0
Three years before current fiscal year		0	0
Four years before current fiscal year		8	0
Prior		50	44
Revolving Loans		40	40
Revolving Loans Converted to Term Loans		0	0
Total loans		98	84
Commercial Loans [Member] | Substandard [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		0	1
Fiscal year before current fiscal year		5	230
Two years before current fiscal year		448	73
Three years before current fiscal year		32	192
Four years before current fiscal year		199	31
Prior		569	468
Revolving Loans		721	912
Revolving Loans Converted to Term Loans		5	1
Total loans		1,979	1,908
Commercial Loans [Member] | Substandard [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		0	0
Fiscal year before current fiscal year		0	186
Two years before current fiscal year		415	0
Three years before current fiscal year			169
Four years before current fiscal year		168	0
Prior		184	86
Revolving Loans		0	0
Revolving Loans Converted to Term Loans		0	0
Total loans		767	441
Commercial Loans [Member] | Substandard [Member] | Business and Corporate Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		0	1
Fiscal year before current fiscal year		5	44
Two years before current fiscal year		33	25
Three years before current fiscal year		32	23
Four years before current fiscal year		31	31
Prior		224	181
Revolving Loans		389	435
Revolving Loans Converted to Term Loans		5	1
Total loans		719	741
Commercial Loans [Member] | Substandard [Member] | Global Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		0	0
Fiscal year before current fiscal year		0	0
Two years before current fiscal year		0	48
Three years before current fiscal year		0	0
Four years before current fiscal year		0	0
Prior		91	131
Revolving Loans		332	477
Revolving Loans Converted to Term Loans		0	0
Total loans		423	656
Commercial Loans [Member] | Substandard [Member] | Other Commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		0	0
Fiscal year before current fiscal year		0	0
Two years before current fiscal year		0	0
Three years before current fiscal year		0	0
Four years before current fiscal year		0	0
Prior		70	70
Revolving Loans		0	0
Revolving Loans Converted to Term Loans		0	0
Total loans		70	70
Commercial Loans [Member] | Doubtful [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		0	0
Fiscal year before current fiscal year		0	0
Two years before current fiscal year		0	0
Three years before current fiscal year		0	0
Four years before current fiscal year		0	0
Prior		185	123
Revolving Loans		111	217
Revolving Loans Converted to Term Loans		0	0
Total loans		296	340
Commercial Loans [Member] | Doubtful [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		0	0
Fiscal year before current fiscal year		0	0
Two years before current fiscal year		0	0
Three years before current fiscal year		0	0
Four years before current fiscal year		0	0
Prior		0	0
Revolving Loans		0	0
Revolving Loans Converted to Term Loans		0	0
Total loans		0	0
Commercial Loans [Member] | Doubtful [Member] | Business and Corporate Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		0	0
Fiscal year before current fiscal year		0	0
Two years before current fiscal year		0	0
Three years before current fiscal year		0	0
Four years before current fiscal year		0	0
Prior		47	41
Revolving Loans		55	57
Revolving Loans Converted to Term Loans		0	0
Total loans		102	98
Commercial Loans [Member] | Doubtful [Member] | Global Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		0	0
Fiscal year before current fiscal year		0	0
Two years before current fiscal year		0	0
Three years before current fiscal year		0	0
Four years before current fiscal year		0	0
Prior		138	82
Revolving Loans		56	160
Revolving Loans Converted to Term Loans		0	0
Total loans		194	242
Commercial Loans [Member] | Doubtful [Member] | Other Commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		0	0
Fiscal year before current fiscal year		0	0
Two years before current fiscal year		0	0
Three years before current fiscal year		0	0
Four years before current fiscal year		0	0
Prior		0	0
Revolving Loans		0	0
Revolving Loans Converted to Term Loans		0	0
Total loans		$ 0	$ 0

[1] Includes loans funded under the Paycheck Protection Program ("PPP") which totaled $1,294 million and $1,043 million at March 31, 2021 and December 31, 2020, respectively. PPP loans are fully guaranteed by the Small Business Administration, if certain conditions are met.

[2] Represents large multinational firms including globally focused U.S. corporate and financial institutions, U.S. dollar lending to multinational banking clients managed by HSBC on a global basis and complex large business clients supported by Global Banking and Markets relationship managers.