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		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017		Sep. 30, 2016		Sep. 30, 2017		Dec. 31, 2016		Sep. 30, 2016
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period		$ 848	$ 1,063	$ 1,017		$ 912
Provision charged (credited) to income		(22)	62	(120)		353
Charge offs		(35)	(79)	(132)		(238)
Recoveries		8	14	34		33
Net (charge offs) recoveries		(27)	(65)	(98)		(205)
Balance at end of period		799	1,060	799		1,060
Ending balance: collectively evaluated for impairment								$ 576				$ 706
Ending balance: individually evaluated for impairment								223				354
Total allowances for credit losses		848	1,063	1,017		912		799		$ 1,017		1,060
Collectively evaluated for impairment	[1]							65,677				75,439
Individually evaluated for impairment	[2]							919				1,206
Loans carried at the lower of amortized cost or fair value less cost to sell								997				980
Total loans								67,593		73,875		77,625
Commercial Loans [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Total loans								48,128		54,216
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period		81	92	92		86
Provision charged (credited) to income		(1)	(8)	(9)		(2)
Charge offs		(4)	0	(7)		0
Recoveries		0	6	0		6
Net (charge offs) recoveries		(4)	6	(7)		6
Balance at end of period		76	90	76		90
Ending balance: collectively evaluated for impairment								75				88
Ending balance: individually evaluated for impairment								1				2
Total allowances for credit losses		81	92	92		86		76		92		90
Collectively evaluated for impairment	[1]							10,425				10,899
Individually evaluated for impairment	[2]							17				59
Loans carried at the lower of amortized cost or fair value less cost to sell								0				0
Total loans								10,442		10,890		10,958
Commercial Loans [Member] | Business and Corporate Banking [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period		261	331	317	[3]	407	[3]
Provision charged (credited) to income		(9)	13	(56)	[3]	(29)	[3]
Charge offs		(7)	(26)	(30)	[3]	(65)	[3]
Recoveries		2	3	16	[3]	8	[3]
Net (charge offs) recoveries		(5)	(23)	(14)	[3]	(57)	[3]
Balance at end of period	[3]	247	321	247		321
Ending balance: collectively evaluated for impairment	[3]							196				253
Ending balance: individually evaluated for impairment	[3]							51				68
Total allowances for credit losses		261	331	317	[3]	407	[3]	247	[3]	317	[3]	321	[3]
Collectively evaluated for impairment	[1],[3]							13,008				13,658
Individually evaluated for impairment	[2],[3]							320				399
Loans carried at the lower of amortized cost or fair value less cost to sell	[3]							0				0
Total loans								13,328		14,080		14,057	[3]
Commercial Loans [Member] | Global Banking [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period		425	515	508	[3]	267	[3]
Provision charged (credited) to income		(21)	38	(60)	[3]	371	[3]
Charge offs		(14)	(7)	(59)	[3]	(92)	[3]
Recoveries		0	0	1	[3]	0	[3]
Net (charge offs) recoveries		(14)	(7)	(58)	[3]	(92)	[3]
Balance at end of period	[3]	390	546	390		546
Ending balance: collectively evaluated for impairment	[3]							228				275
Ending balance: individually evaluated for impairment	[3]							162				271
Total allowances for credit losses		425	515	508	[3]	267	[3]	390	[3]	508	[3]	546	[3]
Collectively evaluated for impairment	[1],[3]							19,434				24,889
Individually evaluated for impairment	[2],[3]							516				670
Loans carried at the lower of amortized cost or fair value less cost to sell	[3]							0				0
Total loans								19,950	[4],[5]	23,481	[4],[5]	25,559	[3]
Commercial Loans [Member] | Other Commercial [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period		14	15	13		19
Provision charged (credited) to income		(1)	(1)	1		(5)
Charge offs		0	0	(1)		0
Recoveries		0	0	0		0
Net (charge offs) recoveries		0	0	(1)		0
Balance at end of period		13	14	13		14
Ending balance: collectively evaluated for impairment								13				13
Ending balance: individually evaluated for impairment								0				1
Total allowances for credit losses		14	15	13		19		13		13		14
Collectively evaluated for impairment	[1]							4,408				7,451
Individually evaluated for impairment	[2]							0				7
Loans carried at the lower of amortized cost or fair value less cost to sell								0				0
Total loans								4,408	[4]	5,765	[4]	7,458
Consumer Loans [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Total loans								19,465		19,659
Consumer Loans [Member] | Residential Mortgages [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period		20	50	26		68
Provision charged (credited) to income		4	11	(4)		(5)
Charge offs		0	(34)	(3)		(43)
Recoveries		2	3	7		10
Net (charge offs) recoveries		2	(31)	4		(33)
Balance at end of period		26	30	26		30
Ending balance: collectively evaluated for impairment								19				20
Ending balance: individually evaluated for impairment								7				10
Total allowances for credit losses		20	50	26		68		26		26		30
Collectively evaluated for impairment	[1]							16,207				16,121
Individually evaluated for impairment	[2]							59				62
Loans carried at the lower of amortized cost or fair value less cost to sell								930				908
Total loans								17,196		17,181		17,091
Consumer Loans [Member] | Home Equity Mortgages [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period		12	19	20		24
Provision charged (credited) to income		(2)	1	(8)		(2)
Charge offs		(1)	(4)	(5)		(9)
Recoveries		1	1	3		4
Net (charge offs) recoveries		0	(3)	(2)		(5)
Balance at end of period		10	17	10		17
Ending balance: collectively evaluated for impairment								9				16
Ending balance: individually evaluated for impairment								1				1
Total allowances for credit losses		12	19	20		24		10		20		17
Collectively evaluated for impairment	[1]							1,164				1,378
Individually evaluated for impairment	[2]							3				4
Loans carried at the lower of amortized cost or fair value less cost to sell								67				72
Total loans								1,234		1,408		1,454
Consumer Loans [Member] | Credit Cards [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period		30	33	34		32
Provision charged (credited) to income		8	7	17		21
Charge offs		(8)	(7)	(24)		(23)
Recoveries		2	1	5		4
Net (charge offs) recoveries		(6)	(6)	(19)		(19)
Balance at end of period		32	34	32		34
Ending balance: collectively evaluated for impairment								31				33
Ending balance: individually evaluated for impairment								1				1
Total allowances for credit losses		30	33	34		32		32		34		34
Collectively evaluated for impairment	[1]							652				663
Individually evaluated for impairment	[2]							4				5
Loans carried at the lower of amortized cost or fair value less cost to sell								0				0
Total loans								656		688		668
Consumer Loans [Member] | Other consumer [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period		5	8	7		9
Provision charged (credited) to income		0	1	(1)		4
Charge offs		(1)	(1)	(3)		(6)
Recoveries		1	0	2		1
Net (charge offs) recoveries		0	(1)	(1)		(5)
Balance at end of period		5	8	5		8
Ending balance: collectively evaluated for impairment								5				8
Ending balance: individually evaluated for impairment								0				0
Total allowances for credit losses		$ 5	$ 8	$ 7		$ 9		5		7		8
Collectively evaluated for impairment	[1]							379				380
Individually evaluated for impairment	[2]							0				0
Loans carried at the lower of amortized cost or fair value less cost to sell								0				0
Total loans								$ 379		$ 382		$ 380

[1] During the first quarter of 2017, in conjunction with the creation of the new Corporate Center segment as discussed further in Note 14, "Business Segments," we reclassified loans to HSBC affiliates from global banking to other commercial and revised the prior period to conform with the current year presentation. As a result, other commercial includes loans to HSBC affiliates totaling $1,820 million and $4,463 million at September 30, 2017 and 2016, respectively, for which we do not carry an associated allowance for credit losses.

[2] 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 $662 million at both September 30, 2017 and September 30, 2016

[3] During the fourth quarter of 2016, we transferred certain client relationships from Commercial Banking to Global Banking and Markets as discussed further in Note 14, "Business Segments." As a result, we reclassified $3.6 billion of loans and $21 million of allowance for credit losses from business and corporate banking to global banking at September 30, 2016 to conform with the current year presentation

[4] During the first quarter of 2017, in conjunction with the creation of the new Corporate Center segment as discussed further in Note 14, "Business Segments," we reclassified loans to HSBC affiliates from global banking to other commercial and revised the prior period to conform with the current year presentation. As a result, other commercial includes loans to HSBC affiliates which totaled $1,820 million and $3,274 million at September 30, 2017 and December 31, 2016, respectively. All tables below have been restated to reflect this reclassification, as applicable. See Note 13, "Related Party Transactions," for additional information regarding loans to HSBC affiliates.

[5] 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.