Allowance for Credit Losses - Summary of Changes in Allowance for Credit Losses and Related Loan Balance by Product (Detail) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016		Dec. 31, 2015	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period					$ 1,017				$ 912	$ 1,017	$ 912		$ 680
Provision charged (credited) to income		$ (45)	$ (22)	$ (21)	(77)	$ 19	$ 62	$ 134	157	(165)	372	[1]	361
Charge offs										(230)	(309)	[1],[2]	(167)
Recoveries										59	42		38
Net (charge offs) recoveries										(171)	(267)		(129)
Balance at end of period		681				1,017				681	1,017		912
Ending balance: collectively evaluated for impairment														$ 545		$ 698		$ 823
Ending balance: individually evaluated for impairment														136		319		89
Total allowances for credit losses		681			1,017	1,017			912	1,017	912		680	681		1,017		912
Collectively evaluated for impairment	[3]													70,826		71,728		80,736
Individually evaluated for impairment	[4]													763		1,121		660
Loans carried at the lower of amortized cost or fair value less cost to sell														974		1,026		1,521
Total loans														72,563		73,875		82,917
Commercial Loans [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Total loans														53,035		54,216
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period					92				86	92	86		89
Provision charged (credited) to income										(3)	0	[1]	2
Charge offs										(7)	(1)	[1],[2]	(10)
Recoveries										0	7		5
Net (charge offs) recoveries										(7)	6		(5)
Balance at end of period		82				92				82	92		86
Ending balance: collectively evaluated for impairment														82		91		85
Ending balance: individually evaluated for impairment														0		1		1
Total allowances for credit losses		82			92	92			86	92	86		89	82		92		86
Collectively evaluated for impairment	[3]													10,522		10,847		9,890
Individually evaluated for impairment	[4]													11		43		110
Loans carried at the lower of amortized cost or fair value less cost to sell														0		0		0
Total loans														10,533		10,890		10,000
Commercial Loans [Member] | Business and Corporate Banking [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period					317				407	317	407		251
Provision charged (credited) to income										(59)	10	[1]	215
Charge offs										(37)	(110)	[1],[2]	(69)
Recoveries										23	10		10
Net (charge offs) recoveries										(14)	(100)		(59)
Balance at end of period		244				317				244	317		407
Ending balance: collectively evaluated for impairment														199		262		355
Ending balance: individually evaluated for impairment														45		55		52
Total allowances for credit losses		244			317	317			407	317	407		251	244		317		407
Collectively evaluated for impairment	[3]													12,254		13,738		14,148
Individually evaluated for impairment	[4]													250		342		217
Loans carried at the lower of amortized cost or fair value less cost to sell														0		0		0
Total loans														12,504		14,080		14,365
Commercial Loans [Member] | Global Banking [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period					508				267	508	267		131
Provision charged (credited) to income										(116)	348	[1]	136
Charge offs										(141)	(107)	[1],[2]	0
Recoveries										13	0		0
Net (charge offs) recoveries										(128)	(107)		0
Balance at end of period		264				508				264	508		267
Ending balance: collectively evaluated for impairment														182		257		267
Ending balance: individually evaluated for impairment														82		251		0
Total allowances for credit losses		264			508	508			267	508	267		131	264		508		267
Collectively evaluated for impairment	[3]													19,651		22,820		29,786
Individually evaluated for impairment	[4]													437		661		119
Loans carried at the lower of amortized cost or fair value less cost to sell														0		0		0
Total loans														20,088	[5],[6]	23,481	[5],[6]	29,905
Commercial Loans [Member] | Other Commercial [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period					13				19	13	19		21
Provision charged (credited) to income										6	(6)	[1]	(2)
Charge offs										(1)	0	[1],[2]	(1)
Recoveries										0	0		1
Net (charge offs) recoveries										(1)	0		0
Balance at end of period		18				13				18	13		19
Ending balance: collectively evaluated for impairment														18		12		18
Ending balance: individually evaluated for impairment														0		1		1
Total allowances for credit losses		18			13	13			19	13	19		21	18		13		19
Collectively evaluated for impairment	[3]													9,910		5,758		8,176
Individually evaluated for impairment	[4]													0		7		7
Loans carried at the lower of amortized cost or fair value less cost to sell														0		0		0
Total loans														9,910	[5]	5,765	[5]	8,183
Consumer Loans [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Total loans														19,528		19,659
Consumer Loans [Member] | Residential Mortgages [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period					26				68	26	68		107
Provision charged (credited) to income										(7)	(9)	[1]	(15)
Charge offs										(4)	(45)	[1],[2]	(35)
Recoveries										10	12		11
Net (charge offs) recoveries										6	(33)		(24)
Balance at end of period		25				26				25	26		68
Ending balance: collectively evaluated for impairment														18		17		35
Ending balance: individually evaluated for impairment														7		9		33
Total allowances for credit losses		25			26	26			68	26	68		107	25		26		68
Collectively evaluated for impairment	[3]													16,308		16,165		16,112
Individually evaluated for impairment	[4]													57		60		197
Loans carried at the lower of amortized cost or fair value less cost to sell														908		956		1,449
Total loans														17,273		17,181		17,758
Consumer Loans [Member] | Home Equity Mortgages [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period					20				24	20	24		32
Provision charged (credited) to income										(8)	(1)	[1]	(4)
Charge offs										(6)	(8)	[1],[2]	(8)
Recoveries										5	5		4
Net (charge offs) recoveries										(1)	(3)		(4)
Balance at end of period		11				20				11	20		24
Ending balance: collectively evaluated for impairment														10		19		23
Ending balance: individually evaluated for impairment														1		1		1
Total allowances for credit losses		11			20	20			24	20	24		32	11		20		24
Collectively evaluated for impairment	[3]													1,121		1,335		1,523
Individually evaluated for impairment	[4]													4		3		5
Loans carried at the lower of amortized cost or fair value less cost to sell														66		70		72
Total loans														1,191	[7]	1,408		1,600
Consumer Loans [Member] | Credit Cards [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period					34				32	34	32		39
Provision charged (credited) to income										22	26	[1]	20
Charge offs										(30)	(30)	[1],[2]	(32)
Recoveries										6	6		5
Net (charge offs) recoveries										(24)	(24)		(27)
Balance at end of period		32				34				32	34		32
Ending balance: collectively evaluated for impairment														31		33		31
Ending balance: individually evaluated for impairment														1		1		1
Total allowances for credit losses		32			34	34			32	34	32		39	32		34		32
Collectively evaluated for impairment	[3]													717		683		694
Individually evaluated for impairment	[4]													4		5		5
Loans carried at the lower of amortized cost or fair value less cost to sell														0		0		0
Total loans														721	[8]	688		699
Consumer Loans [Member] | Other consumer [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period					7				9	7	9		10
Provision charged (credited) to income										0	4	[1]	9
Charge offs										(4)	(8)	[1],[2]	(12)
Recoveries										2	2		2
Net (charge offs) recoveries										(2)	(6)		(10)
Balance at end of period		5				7				5	7		9
Ending balance: collectively evaluated for impairment														5		7		9
Ending balance: individually evaluated for impairment														0		0		0
Total allowances for credit losses		$ 5			$ 7	$ 7			$ 9	$ 7	$ 9		$ 10	5		7		9
Collectively evaluated for impairment	[3]													343		382		407
Individually evaluated for impairment	[4]													0		0		0
Loans carried at the lower of amortized cost or fair value less cost to sell														0		0		0
Total loans														$ 343		$ 382		$ 407

[1] The provision for credit losses and charge-offs for residential mortgage loans during 2016 includes $11 million related to the lower of amortized cost or fair value adjustment attributable to credit factors for loans transferred to held for sale. See Note 7, "Loans Held for Sale," for additional information.

[2] For collateral dependent loans that are transferred to held for sale, the existing allowance for credit losses at the time of transfer are recognized as a charge-off. We transferred to held for sale certain residential mortgage loans during 2016 and, accordingly, we recognized the existing allowance for credit losses on these loans as additional charge-offs totaling $22 million during 2016.

[3] During 2017, in conjunction with the creation of the new Corporate Center segment as discussed further in Note 22, "Business Segments," we reclassified loans to HSBC affiliates from global banking to other commercial and revised prior periods to conform with the current year presentation. As a result, other commercial includes loans to HSBC affiliates totaling $6,750 million, $3,274 million and $4,815 million at December 31, 2017, 2016 and 2015, respectively, for which we do not carry an associated allowance for credit losses.

[4] For consumer loans and certain small business loans, these amounts represent TDR Loans for which we evaluate reserves using a discounted cash flow methodology. Each loan is individually identified as a TDR Loan and then grouped together with other TDR Loans with similar characteristics. The discounted cash flow analysis is then applied to these groups of TDR Loans. Loans individually evaluated for impairment exclude TDR Loans that are carried at the lower of amortized cost or fair value of the collateral less cost to sell which totaled $655 million, $672 million and $881 million at December 31, 2017, 2016 and 2015, respectively.

[5] During 2017, in conjunction with the creation of the new Corporate Center segment as discussed further in Note 22, "Business Segments," we reclassified loans to HSBC affiliates from global banking to other commercial and revised prior periods to conform with the current year presentation. As a result, other commercial includes loans to HSBC affiliates which totaled $6,750 million and $3,274 million at December 31, 2017 and 2016, respectively. All tables below have been restated to reflect this reclassification, as applicable. See Note 21, "Related Party Transactions," for additional information regarding loans to HSBC affiliates.

[6] 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.

[7] Home equity mortgage maturities reflect estimates based on historical payment patterns.

[8] As credit card receivables do not have stated maturities, the table reflects estimates based on historical payment patterns.