Loans - Status of Loan Portfolios (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] | Performing Loans [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		928		2,509
Fiscal year before current fiscal year		2,406		5,307
Two years before current fiscal year		4,273		3,433
Three years before current fiscal year		2,849		1,979
Four years before current fiscal year		1,334		638
Prior		12,029		11,732
Revolving Loans		14,423		15,115
Revolving Loans, Converted to Term Loans		272		342
Total loans		38,514		41,055
Commercial Loans [Member] | Performing 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		2,999		2,775
Three years before current fiscal year		2,217		1,368
Four years before current fiscal year		783		264
Prior		1,416		1,594
Revolving Loans		49		79
Revolving Loans, Converted to Term Loans		16		20
Total loans		8,122		10,420
Commercial Loans [Member] | Performing Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		304		1,079
Fiscal year before current fiscal year		647		606
Two years before current fiscal year		527		253
Three years before current fiscal year		210		275
Four years before current fiscal year		217		151
Prior		4,842		3,485
Revolving Loans		5,645		7,145
Revolving Loans, Converted to Term Loans		256		322
Total loans		12,648		13,316
Commercial Loans [Member] | Performing 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		4,630		6,023
Revolving Loans		4,823		5,632
Revolving Loans, Converted to Term Loans		0		0
Total loans		11,308		13,182
Commercial Loans [Member] | Performing 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] | Nonaccrual Loans [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		0		1
Fiscal year before current fiscal year		4		15
Two years before current fiscal year		95		36
Three years before current fiscal year		56		60
Four years before current fiscal year		54		1
Prior		64		150
Revolving Loans		126		281
Revolving Loans, Converted to Term Loans		0		0
Total loans		399		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		81		24
Three years before current fiscal year		26		0
Four years before current fiscal year		0		0
Prior		21		20
Revolving Loans		0		0
Revolving Loans, Converted to Term Loans		0		0
Total loans		128		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		4		15
Two years before current fiscal year		14		12
Three years before current fiscal year		30		60
Four years before current fiscal year		54		1
Prior		1		3
Revolving Loans		36		71
Revolving Loans, Converted to Term Loans		0		0
Total loans		139		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		42		127
Revolving Loans		90		210
Revolving Loans, Converted to Term Loans		0		0
Total loans		132		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		1		0
Revolving Loans, Converted to Term Loans		0		0
Total loans		1		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		1		0
Revolving Loans, Converted to Term Loans		0		0
Total loans		1		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		3,598		4,666
Fiscal year before current fiscal year		3,613		2,566
Two years before current fiscal year		1,785		1,465
Three years before current fiscal year		971		1,694
Four years before current fiscal year		1,096		1,985
Prior		5,547		6,991
Revolving Loans		2		1,122
Total loans	[3]	16,612	[4]	20,489
Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		3,580		4,529
Fiscal year before current fiscal year		3,566		2,476
Two years before current fiscal year		1,751		1,431
Three years before current fiscal year		951		1,658
Four years before current fiscal year		1,076		1,930
Prior		5,169		6,353
Revolving Loans		0		0
Total loans		16,093		18,377
Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		12		50
Fiscal year before current fiscal year		34		51
Two years before current fiscal year		29		34
Three years before current fiscal year		19		35
Four years before current fiscal year		19		47
Prior		256		510
Revolving Loans		0		0
Total loans		369		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		0		1,066
Total loans		0		1,066
Consumer Loans [Member] | Other consumer [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		6		87
Fiscal year before current fiscal year		13		39
Two years before current fiscal year		5		0
Three years before current fiscal year		1		1
Four years before current fiscal year		1		8
Prior		122		128
Revolving Loans		2		56
Total loans	[5]	150		319
Consumer Loans [Member] | Performing Loans [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		3,592		4,628
Fiscal year before current fiscal year		3,574		2,459
Two years before current fiscal year		1,710		1,371
Three years before current fiscal year		905		1,607
Four years before current fiscal year		1,036		1,905
Prior		4,961		6,255
Revolving Loans		2		1,101
Total loans		15,780		19,326
Consumer Loans [Member] | Performing Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		3,574		4,491
Fiscal year before current fiscal year		3,527		2,369
Two years before current fiscal year		1,677		1,338
Three years before current fiscal year		885		1,572
Four years before current fiscal year		1,016		1,852
Prior		4,629		5,676
Revolving Loans		0		0
Total loans		15,308		17,298
Consumer Loans [Member] | Performing Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		12		50
Fiscal year before current fiscal year		34		51
Two years before current fiscal year		28		33
Three years before current fiscal year		19		34
Four years before current fiscal year		19		45
Prior		210		451
Revolving Loans		0		0
Total loans		322		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		0		1,047
Total loans		0		1,047
Consumer Loans [Member] | Performing Loans [Member] | Other consumer [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		6		87
Fiscal year before current fiscal year		13		39
Two years before current fiscal year		5		0
Three years before current fiscal year		1		1
Four years before current fiscal year		1		8
Prior		122		128
Revolving Loans		2		54
Total loans		150		317
Consumer Loans [Member] | Nonaccrual Loans [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		6		38
Fiscal year before current fiscal year		39		107
Two years before current fiscal year		75		94
Three years before current fiscal year		66		87
Four years before current fiscal year		60		80
Prior		586		736
Revolving Loans		0		0
Total loans		832		1,142
Consumer Loans [Member] | Nonaccrual Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Current fiscal year		6		38
Fiscal year before current fiscal year		39		107
Two years before current fiscal year		74		93
Three years before current fiscal year		66		86
Four years before current fiscal year		60		78
Prior		540		677
Revolving Loans		0		0
Total loans		785		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		0		1
Four years before current fiscal year		0		2
Prior		46		59
Revolving Loans		0		0
Total loans		47		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		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		21
Total loans		0		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		0		19
Total loans		0		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		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		2
Total loans		$ 0		$ 2

[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.

[3] The decrease in loans at September 30, 2021 includes the impact of our decision to exit our mass market retail banking business as well as our remaining retail credit card portfolio which resulted in the transfer of certain loans to held for sale during the second quarter of 2021. See Note 8, "Loans Held for Sale," for additional information.

[4] The decrease in past due loans at September 30, 2021 includes the impact of our decision to exit our mass market retail banking business as well as our remaining retail credit card portfolio which resulted in the transfer of certain loans to held for sale during the second quarter of 2021, including $115 million of consumer loans which were past due 30 days or more.

[5] Includes certain student loans that we have elected to designate under the fair value option and are therefore carried at fair value, which totaled $27 million and $32 million at September 30, 2021 and December 31, 2020, respectively. See Note 11, "Fair Value Option," for further details.