Loans - Credit Risk Profile of 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] | Investment Grade [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		138	1,517
Fiscal year before current fiscal year		1,669	1,910
Two years before current fiscal year		1,569	1,017
Three years before current fiscal year		1,285	688
Four years before current fiscal year		719	348
Prior		7,167	7,269
Revolving Loans		8,795	8,718
Revolving Loans Converted to Term Loans		36	47
Total loans		21,378	21,514
Commercial Loans [Member] | Investment Grade [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		0	339
Fiscal year before current fiscal year		339	1,123
Two years before current fiscal year		917	817
Three years before current fiscal year		979	318
Four years before current fiscal year		342	179
Prior		542	640
Revolving Loans		5	6
Revolving Loans Converted to Term Loans		0	0
Total loans		3,124	3,422
Commercial Loans [Member] | Investment Grade [Member] | Business and Corporate Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		16	342
Fiscal year before current fiscal year		340	147
Two years before current fiscal year		176	37
Three years before current fiscal year		40	34
Four years before current fiscal year		30	23
Prior		1,820	1,486
Revolving Loans		2,145	2,499
Revolving Loans Converted to Term Loans		36	47
Total loans		4,603	4,615
Commercial Loans [Member] | Investment Grade [Member] | Global Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		117	464
Fiscal year before current fiscal year		567	477
Two years before current fiscal year		331	46
Three years before current fiscal year		52	231
Four years before current fiscal year		234	30
Prior		4,146	4,618
Revolving Loans		4,044	4,281
Revolving Loans Converted to Term Loans		0	0
Total loans		9,491	10,147
Commercial Loans [Member] | Investment Grade [Member] | Other Commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		5	372
Fiscal year before current fiscal year		423	163
Two years before current fiscal year		145	117
Three years before current fiscal year		214	105
Four years before current fiscal year		113	116
Prior		659	525
Revolving Loans		2,601	1,932
Revolving Loans Converted to Term Loans		0	0
Total loans		4,160	3,330
Commercial Loans [Member] | Noninvestment Grade [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		69	993
Fiscal year before current fiscal year		1,139	3,412
Two years before current fiscal year		3,491	2,452
Three years before current fiscal year		2,161	1,351
Four years before current fiscal year		1,250	291
Prior		5,710	4,613
Revolving Loans		5,666	6,678
Revolving Loans Converted to Term Loans		288	295
Total loans		19,774	20,085
Commercial Loans [Member] | Noninvestment Grade [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		0	206
Fiscal year before current fiscal year		224	2,652
Two years before current fiscal year		2,746	1,982
Three years before current fiscal year		1,806	1,050
Four years before current fiscal year		958	85
Prior		1,161	974
Revolving Loans		51	73
Revolving Loans Converted to Term Loans		17	20
Total loans		6,963	7,042
Commercial Loans [Member] | Noninvestment Grade [Member] | Business and Corporate Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		30	738
Fiscal year before current fiscal year		864	474
Two years before current fiscal year		461	228
Three years before current fiscal year		207	301
Four years before current fiscal year		288	129
Prior		2,256	2,002
Revolving Loans		4,145	4,717
Revolving Loans Converted to Term Loans		271	275
Total loans		8,522	8,864
Commercial Loans [Member] | Noninvestment Grade [Member] | Global Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		0	43
Fiscal year before current fiscal year		38	18
Two years before current fiscal year		27	144
Three years before current fiscal year		147	0
Four years before current fiscal year		4	74
Prior		2,172	1,532
Revolving Loans		1,278	1,561
Revolving Loans Converted to Term Loans		0	0
Total loans		3,666	3,372
Commercial Loans [Member] | Noninvestment Grade [Member] | Other Commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		39	6
Fiscal year before current fiscal year		13	268
Two years before current fiscal year		257	98
Three years before current fiscal year		1	0
Four years before current fiscal year		0	3
Prior		121	105
Revolving Loans		192	327
Revolving Loans Converted to Term Loans		0	0
Total loans		$ 623	$ 807

[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.