Loans - Status of Loan Portfolios (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] | Performing Loans [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		207	2,509
Fiscal year before current fiscal year		2,806	5,307
Two years before current fiscal year		5,045	3,433
Three years before current fiscal year		3,397	1,979
Four years before current fiscal year		1,912	638
Prior		12,728	11,732
Revolving Loans		14,257	15,115
Revolving Loans Converted to Term Loans		322	342
Total loans		40,674	41,055
Commercial Loans [Member] | Performing 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	2,775
Three years before current fiscal year		2,753	1,368
Four years before current fiscal year		1,300	264
Prior		1,681	1,594
Revolving Loans		56	79
Revolving Loans Converted to Term Loans		17	20
Total loans		10,033	10,420
Commercial Loans [Member] | Performing Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		46	1,079
Fiscal year before current fiscal year		1,202	606
Two years before current fiscal year		622	253
Three years before current fiscal year		230	275
Four years before current fiscal year		261	151
Prior		4,074	3,485
Revolving Loans		6,213	7,145
Revolving Loans Converted to Term Loans		305	322
Total loans		12,953	13,316
Commercial Loans [Member] | Performing 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		6,193	6,023
Revolving Loans		5,195	5,632
Revolving Loans Converted to Term Loans		0	0
Total loans		12,905	13,182
Commercial Loans [Member] | Performing 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] | Nonaccrual Loans [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		0	1
Fiscal year before current fiscal year		2	15
Two years before current fiscal year		15	36
Three years before current fiscal year		49	60
Four years before current fiscal year		57	1
Prior		149	150
Revolving Loans		204	281
Revolving Loans Converted to Term Loans		2	0
Total loans		478	544
Commercial Loans [Member] | Nonaccrual Loans [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	24
Three years before current fiscal year		32	0
Four years before current fiscal year		0	0
Prior		22	20
Revolving Loans		0	0
Revolving Loans Converted to Term Loans		0	0
Total loans		54	44
Commercial Loans [Member] | Nonaccrual Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		0	1
Fiscal year before current fiscal year		2	15
Two years before current fiscal year		15	12
Three years before current fiscal year		17	60
Four years before current fiscal year		57	1
Prior		2	3
Revolving Loans		77	71
Revolving Loans Converted to Term Loans		2	0
Total loans		172	163
Commercial Loans [Member] | Nonaccrual Loans [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		125	127
Revolving Loans		127	210
Revolving Loans Converted to Term Loans		0	0
Total loans		252	337
Commercial Loans [Member] | Nonaccrual Loans [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
Commercial Loans [Member] | Nonperforming Loans, Accruing Loans 90 Days or More Past Due [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] | Nonperforming Loans, Accruing Loans 90 Days or More Past Due [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] | Nonperforming Loans, Accruing Loans 90 Days or More Past Due [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		0	0
Revolving Loans		0	0
Revolving Loans Converted to Term Loans		0	0
Total loans		0	0
Commercial Loans [Member] | Nonperforming Loans, Accruing Loans 90 Days or More Past Due [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	0
Revolving Loans		0	0
Revolving Loans Converted to Term Loans		0	0
Total loans		0	0
Commercial Loans [Member] | Nonperforming Loans, Accruing Loans 90 Days or More Past Due [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
Consumer Loans [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		1,393	4,666
Fiscal year before current fiscal year		4,420	2,566
Two years before current fiscal year		2,343	1,465
Three years before current fiscal year		1,342	1,694
Four years before current fiscal year		1,546	1,985
Prior		8,343	6,991
Revolving Loans		987	1,122
Total loans		20,374	20,489
Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		1,365	4,529
Fiscal year before current fiscal year		4,289	2,476
Two years before current fiscal year		2,260	1,431
Three years before current fiscal year		1,308	1,658
Four years before current fiscal year		1,511	1,930
Prior		7,692	6,353
Revolving Loans		0	0
Total loans		18,425	18,377
Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		3	50
Fiscal year before current fiscal year		51	51
Two years before current fiscal year		50	34
Three years before current fiscal year		33	35
Four years before current fiscal year		34	47
Prior		527	510
Revolving Loans		0	0
Total loans		698	727
Consumer Loans [Member] | Credit Cards [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		937	1,066
Total loans		937	1,066
Consumer Loans [Member] | Other consumer [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		25	87
Fiscal year before current fiscal year		80	39
Two years before current fiscal year		33	0
Three years before current fiscal year		1	1
Four years before current fiscal year		1	8
Prior		124	128
Revolving Loans		50	56
Total loans	[3]	314	319
Consumer Loans [Member] | Performing Loans [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		1,393	4,628
Fiscal year before current fiscal year		4,373	2,459
Two years before current fiscal year		2,226	1,371
Three years before current fiscal year		1,231	1,607
Four years before current fiscal year		1,451	1,905
Prior		7,546	6,255
Revolving Loans		958	1,101
Total loans		19,178	19,326
Consumer Loans [Member] | Performing Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		1,365	4,491
Fiscal year before current fiscal year		4,243	2,369
Two years before current fiscal year		2,145	1,338
Three years before current fiscal year		1,198	1,572
Four years before current fiscal year		1,417	1,852
Prior		6,956	5,676
Revolving Loans		0	0
Total loans		17,324	17,298
Consumer Loans [Member] | Performing Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		3	50
Fiscal year before current fiscal year		51	51
Two years before current fiscal year		49	33
Three years before current fiscal year		32	34
Four years before current fiscal year		33	45
Prior		466	451
Revolving Loans		0	0
Total loans		634	664
Consumer Loans [Member] | Performing Loans [Member] | Credit Cards [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		909	1,047
Total loans		909	1,047
Consumer Loans [Member] | Performing Loans [Member] | Other consumer [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		25	87
Fiscal year before current fiscal year		79	39
Two years before current fiscal year		32	0
Three years before current fiscal year		1	1
Four years before current fiscal year		1	8
Prior		124	128
Revolving Loans		49	54
Total loans		311	317
Consumer Loans [Member] | Nonaccrual Loans [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		0	38
Fiscal year before current fiscal year		46	107
Two years before current fiscal year		116	94
Three years before current fiscal year		111	87
Four years before current fiscal year		95	80
Prior		797	736
Revolving Loans		0	0
Total loans		1,165	1,142
Consumer Loans [Member] | Nonaccrual Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		0	38
Fiscal year before current fiscal year		46	107
Two years before current fiscal year		115	93
Three years before current fiscal year		110	86
Four years before current fiscal year		94	78
Prior		736	677
Revolving Loans		0	0
Total loans		1,101	1,079
Consumer Loans [Member] | Nonaccrual Loans [Member] | Home Equity Mortgages [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		1	1
Three years before current fiscal year		1	1
Four years before current fiscal year		1	2
Prior		61	59
Revolving Loans		0	0
Total loans		64	63
Consumer Loans [Member] | Nonperforming Loans, Accruing Loans 90 Days or More Past Due [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		0	0
Fiscal year before current fiscal year		1	0
Two years before current fiscal year		1	0
Three years before current fiscal year		0	0
Four years before current fiscal year		0	0
Prior		0	0
Revolving Loans		29	21
Total loans		31	21
Consumer Loans [Member] | Nonperforming Loans, Accruing Loans 90 Days or More Past Due [Member] | Credit Cards [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		28	19
Total loans		28	19
Consumer Loans [Member] | Nonperforming Loans, Accruing Loans 90 Days or More Past Due [Member] | Other consumer [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		0	0
Fiscal year before current fiscal year		1	0
Two years before current fiscal year		1	0
Three years before current fiscal year		0	0
Four years before current fiscal year		0	0
Prior		0	0
Revolving Loans		1	2
Total loans		$ 3	$ 2

[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.

[3] Includes certain student loans that we have elected to designate under the fair value option and are therefore carried at fair value, which totaled $30 million and $32 million at March 31, 2021 and December 31, 2020, respectively. See Note 10, "Fair Value Option," for further details.