Loans - Credit Risk Profile of Commercial Loans (Detail) - USD ($) $ in Millions		Sep. 30, 2021	Dec. 31, 2020
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans		$ 55,526	$ 62,088
Commercial Loans [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		928	2,510
Fiscal year before current fiscal year		2,410	5,322
Two years before current fiscal year		4,368	3,469
Three years before current fiscal year		2,905	2,039
Four years before current fiscal year		1,388	639
Prior		12,093	11,882
Revolving Loans		14,550	15,396
Revolving Loans, Converted to Term Loans		272	342
Total loans		38,914	41,599
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		7	545
Fiscal year before current fiscal year		635	3,775
Two years before current fiscal year		3,080	2,799
Three years before current fiscal year		2,243	1,368
Four years before current fiscal year		783	264
Prior		1,437	1,614
Revolving Loans		49	79
Revolving Loans, Converted to Term Loans		16	20
Total loans		8,250	10,464
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		304	1,080
Fiscal year before current fiscal year		651	621
Two years before current fiscal year		541	265
Three years before current fiscal year		240	335
Four years before current fiscal year		271	152
Prior		4,843	3,488
Revolving Loans		5,682	7,216
Revolving Loans, Converted to Term Loans		256	322
Total loans	[1]	12,788	13,479
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		403	507
Fiscal year before current fiscal year		674	495
Two years before current fiscal year		327	190
Three years before current fiscal year		217	231
Four years before current fiscal year		234	104
Prior		4,672	6,150
Revolving Loans		4,913	5,842
Revolving Loans, Converted to Term Loans		0	0
Total loans	[2]	11,440	13,519
Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		214	378
Fiscal year before current fiscal year		450	431
Two years before current fiscal year		420	215
Three years before current fiscal year		205	105
Four years before current fiscal year		100	119
Prior		1,141	630
Revolving Loans		3,906	2,259
Revolving Loans, Converted to Term Loans		0	0
Total loans		6,436	4,137
Commercial Loans [Member] | Investment Grade [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		411	1,517
Fiscal year before current fiscal year		1,615	1,910
Two years before current fiscal year		1,235	1,017
Three years before current fiscal year		1,257	688
Four years before current fiscal year		462	348
Prior		7,034	7,269
Revolving Loans		10,390	8,718
Revolving Loans, Converted to Term Loans		69	47
Total loans		22,473	21,514
Commercial Loans [Member] | Investment Grade [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		6	339
Fiscal year before current fiscal year		318	1,123
Two years before current fiscal year		563	817
Three years before current fiscal year		809	318
Four years before current fiscal year		129	179
Prior		238	640
Revolving Loans		15	6
Revolving Loans, Converted to Term Loans		0	0
Total loans		2,078	3,422
Commercial Loans [Member] | Investment Grade [Member] | Business and Corporate Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		31	342
Fiscal year before current fiscal year		263	147
Two years before current fiscal year		196	37
Three years before current fiscal year		42	34
Four years before current fiscal year		23	23
Prior		2,308	1,486
Revolving Loans		2,524	2,499
Revolving Loans, Converted to Term Loans		69	47
Total loans		5,456	4,615
Commercial Loans [Member] | Investment Grade [Member] | Global Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		331	464
Fiscal year before current fiscal year		666	477
Two years before current fiscal year		298	46
Three years before current fiscal year		202	231
Four years before current fiscal year		234	30
Prior		3,683	4,618
Revolving Loans		4,112	4,281
Revolving Loans, Converted to Term Loans		0	0
Total loans		9,526	10,147
Commercial Loans [Member] | Investment Grade [Member] | Other Commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		43	372
Fiscal year before current fiscal year		368	163
Two years before current fiscal year		178	117
Three years before current fiscal year		204	105
Four years before current fiscal year		76	116
Prior		805	525
Revolving Loans		3,739	1,932
Revolving Loans, Converted to Term Loans		0	0
Total loans		5,413	3,330
Commercial Loans [Member] | Noninvestment Grade [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		517	993
Fiscal year before current fiscal year		795	3,412
Two years before current fiscal year		3,133	2,452
Three years before current fiscal year		1,648	1,351
Four years before current fiscal year		926	291
Prior		5,059	4,613
Revolving Loans		4,160	6,678
Revolving Loans, Converted to Term Loans		203	295
Total loans		16,441	20,085
Commercial Loans [Member] | Noninvestment Grade [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		1	206
Fiscal year before current fiscal year		317	2,652
Two years before current fiscal year		2,517	1,982
Three years before current fiscal year		1,434	1,050
Four years before current fiscal year		654	85
Prior		1,199	974
Revolving Loans		34	73
Revolving Loans, Converted to Term Loans		16	20
Total loans		6,172	7,042
Commercial Loans [Member] | Noninvestment Grade [Member] | Business and Corporate Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		273	738
Fiscal year before current fiscal year		388	474
Two years before current fiscal year		345	228
Three years before current fiscal year		198	301
Four years before current fiscal year		248	129
Prior		2,535	2,002
Revolving Loans		3,158	4,717
Revolving Loans, Converted to Term Loans		187	275
Total loans		7,332	8,864
Commercial Loans [Member] | Noninvestment Grade [Member] | Global Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		72	43
Fiscal year before current fiscal year		8	18
Two years before current fiscal year		29	144
Three years before current fiscal year		15	0
Four years before current fiscal year		0	74
Prior		989	1,532
Revolving Loans		801	1,561
Revolving Loans, Converted to Term Loans		0	0
Total loans		1,914	3,372
Commercial Loans [Member] | Noninvestment Grade [Member] | Other Commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		171	6
Fiscal year before current fiscal year		82	268
Two years before current fiscal year		242	98
Three years before current fiscal year		1	0
Four years before current fiscal year		24	3
Prior		336	105
Revolving Loans		167	327
Revolving Loans, Converted to Term Loans		0	0
Total loans		$ 1,023	$ 807

[1] Includes loans funded under the Paycheck Protection Program ("PPP") which totaled $437 million and $1,043 million at September 30, 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.