Loans - Summary of Criticized Assets for 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] | Criticized [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		27	18
Fiscal year before current fiscal year		59	607
Two years before current fiscal year		1,273	231
Three years before current fiscal year		768	395
Four years before current fiscal year		498	126
Prior		1,497	1,604
Revolving Loans		1,113	1,867
Revolving Loans, Converted to Term Loans		29	49
Total loans		5,264	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		1,139	115
Three years before current fiscal year		675	340
Four years before current fiscal year		467	85
Prior		788	523
Revolving Loans		21	0
Revolving Loans, Converted to Term Loans		11	0
Total loans		3,101	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		20	115
Two years before current fiscal year		134	68
Three years before current fiscal year		93	55
Four years before current fiscal year		31	41
Prior		486	612
Revolving Loans		701	1,092
Revolving Loans, Converted to Term Loans		18	49
Total loans		1,483	2,050
Commercial Loans [Member] | Criticized [Member] | Global Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		27	0
Fiscal year before current fiscal year		39	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		145	355
Revolving Loans		351	735
Revolving Loans, Converted to Term Loans		0	0
Total loans		562	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		0	0
Prior		78	114
Revolving Loans		40	40
Revolving Loans, Converted to Term Loans		0	0
Total loans		118	154
Commercial Loans [Member] | Special Mention [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		8	17
Fiscal year before current fiscal year		40	377
Two years before current fiscal year		446	158
Three years before current fiscal year		425	203
Four years before current fiscal year		138	95
Prior		478	1,013
Revolving Loans		280	738
Revolving Loans, Converted to Term Loans		10	48
Total loans		1,825	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		352	115
Three years before current fiscal year		361	171
Four years before current fiscal year		110	85
Prior		225	437
Revolving Loans		0	0
Revolving Loans, Converted to Term Loans		0	0
Total loans		1,048	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		1	71
Two years before current fiscal year		94	43
Three years before current fiscal year		64	32
Four years before current fiscal year		28	10
Prior		206	390
Revolving Loans		226	600
Revolving Loans, Converted to Term Loans		10	48
Total loans		629	1,211
Commercial Loans [Member] | Special Mention [Member] | Global Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		8	0
Fiscal year before current fiscal year		39	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		39	142
Revolving Loans		54	98
Revolving Loans, Converted to Term Loans		0	0
Total loans		140	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		0	0
Prior		8	44
Revolving Loans		0	40
Revolving Loans, Converted to Term Loans		0	0
Total loans		8	84
Commercial Loans [Member] | Substandard [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		19	1
Fiscal year before current fiscal year		19	230
Two years before current fiscal year		746	73
Three years before current fiscal year		321	192
Four years before current fiscal year		360	31
Prior		978	468
Revolving Loans		786	912
Revolving Loans, Converted to Term Loans		19	1
Total loans		3,248	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		706	0
Three years before current fiscal year		312	169
Four years before current fiscal year		357	0
Prior		552	86
Revolving Loans		21	0
Revolving Loans, Converted to Term Loans		11	0
Total loans		1,959	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		19	44
Two years before current fiscal year		40	25
Three years before current fiscal year		9	23
Four years before current fiscal year		3	31
Prior		250	181
Revolving Loans		459	435
Revolving Loans, Converted to Term Loans		8	1
Total loans		788	741
Commercial Loans [Member] | Substandard [Member] | Global Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		19	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		106	131
Revolving Loans		266	477
Revolving Loans, Converted to Term Loans		0	0
Total loans		391	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		40	0
Revolving Loans, Converted to Term Loans		0	0
Total loans		110	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		81	0
Three years before current fiscal year		22	0
Four years before current fiscal year		0	0
Prior		41	123
Revolving Loans		47	217
Revolving Loans, Converted to Term Loans		0	0
Total loans		191	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		81	0
Three years before current fiscal year		2	0
Four years before current fiscal year			0
Prior		11	0
Revolving Loans		0	0
Revolving Loans, Converted to Term Loans		0	0
Total loans		94	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		20	0
Four years before current fiscal year		0	0
Prior		30	41
Revolving Loans		16	57
Revolving Loans, Converted to Term Loans		0	0
Total loans		66	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		0	82
Revolving Loans		31	160
Revolving Loans, Converted to Term Loans		0	0
Total loans		31	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 $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.