Business Segments - Summary on Reconciliation of Results Under Ifrs to Us Gaap (Detail) - USD ($) $ in Millions		3 Months Ended				9 Months Ended
		Sep. 30, 2018		Sep. 30, 2017		Sep. 30, 2018		Sep. 30, 2017		Dec. 31, 2017
Operating Statistics [Line Items]
Group Reporting Basis Net interest income	[1]	$ 645		$ 595		$ 1,890		$ 1,808
Group Reporting Basis Other operating income		413		309		1,330	[2]	1,324	[2]
Group Reporting Basis Total operating income		1,058		904		3,220		3,132
Group Reporting Basis Expected credit losses		(26)		(16)		(222)		(86)
Group Reporting Basis Revenues		1,084		920		3,442		3,218
Group Reporting Basis Operating expenses	[2]	784		818		2,873		2,502	[3]
Group Reporting Basis Profit (Loss) Before Income Tax Expense		300		102		569		716
Group Reporting Basis Total assets		205,623		231,847		205,623		231,847
Group Reporting Basis Total loans, net		65,366	[4]	68,234		65,366	[4]	68,234
Group Reporting Basis Goodwill		1,260		1,260		1,260		1,260
Group Reporting Basis Total deposits		97,646	[4]	97,953		97,646	[4]	97,953
Net interest income	[1]	559		560		1,666		1,739
Other operating income	[5]	486		393		1,536	[2]	1,544	[2]
Total operating income		1,045		953		3,202		3,283
Provision for credit losses		20		(22)		(96)		(120)
Total income		1,025		975		3,298		3,403
Operating expenses	[2]	790		820		2,858		2,488	[3]
Income before income tax		235		155		440		915
Total assets		174,002	[6]	197,886		174,002	[6]	197,886		$ 187,235	[6]
Total loans, net		66,221	[4]	66,794		66,221	[4]	66,794		71,882
Goodwill		1,607		1,607		1,607		1,607		1,607
Total deposits		112,090	[4]	121,840		112,090	[4]	121,840		$ 118,702
Group Reporting Basis Adjustments [Member]
Operating Statistics [Line Items]
Group Reporting Basis Net interest income	[1]	(2)	[7]	(9)	[7]	12	[8]	(38)	[8]
Group Reporting Basis Other operating income		(15)	[7]	61	[7]	(35)	[2],[8]	190	[2],[8]
Group Reporting Basis Total operating income		(17)	[7]	52	[7]	(23)	[8]	152	[8]
Group Reporting Basis Expected credit losses		47	[7]	1	[7]	114	[8]	(47)	[8]
Group Reporting Basis Revenues		(64)	[7]	51	[7]	(137)	[8]	199	[8]
Group Reporting Basis Operating expenses	[2]	1	[7]	(2)	[7]	(8)	[8]	0	[3],[8]
Group Reporting Basis Profit (Loss) Before Income Tax Expense		(65)	[7]	53	[7]	(129)	[8]	199	[8]
Group Reporting Basis Total assets	[8]	(31,621)		(33,961)		(31,621)		(33,961)
Group Reporting Basis Total loans, net	[8]	(829)	[4]	(1,039)		(829)	[4]	(1,039)
Group Reporting Basis Goodwill	[8]	347		347		347		347
Group Reporting Basis Total deposits	[8]	(3,441)	[4]	(3,874)		(3,441)	[4]	(3,874)
Group Reporting Basis Reclassifications [Member]
Operating Statistics [Line Items]
Group Reporting Basis Net interest income	[1]	(84)		(26)		(236)		(31)
Group Reporting Basis Other operating income		88		23		241	[2]	30	[2]
Group Reporting Basis Total operating income		4		(3)		5		(1)
Group Reporting Basis Expected credit losses		(1)		(7)		12		13
Group Reporting Basis Revenues		5		4		(7)		(14)
Group Reporting Basis Operating expenses	[2]	5		4		(7)		(14)	[3]
Group Reporting Basis Profit (Loss) Before Income Tax Expense		0		0		0		0
Group Reporting Basis Total assets		0		0		0		0
Group Reporting Basis Total loans, net		1,684	[4]	(401)		1,684	[4]	(401)
Group Reporting Basis Goodwill		0		0		0		0
Group Reporting Basis Total deposits		17,885	[4]	27,761		17,885	[4]	27,761
Reclassification [Member]
Operating Statistics [Line Items]
Group Reporting Basis Net interest income	[1]	0		0		0		0
Group Reporting Basis Other operating income		0		0		0	[2]	0	[2]
Group Reporting Basis Total operating income		0		0		0		0
Group Reporting Basis Expected credit losses		0		0		0		0
Group Reporting Basis Revenues		0		0		0		0
Group Reporting Basis Operating expenses	[2]	0		0		0		0	[3]
Group Reporting Basis Profit (Loss) Before Income Tax Expense		0		0		0		0
Group Reporting Basis Total assets		0		0		0		0
Group Reporting Basis Total loans, net		0	[4]	0		0	[4]	0
Group Reporting Basis Goodwill		0		0		0		0
Group Reporting Basis Total deposits		0	[4]	0		0	[4]	0
RBWM [Member] | Operating Segments [Member]
Operating Statistics [Line Items]
Group Reporting Basis Net interest income	[1]	223		231		664		665
Group Reporting Basis Other operating income		75		65		239	[2]	451	[2]
Group Reporting Basis Total operating income		298		296		903		1,116
Group Reporting Basis Expected credit losses		10		11		16		17
Group Reporting Basis Revenues		288		285		887		1,099
Group Reporting Basis Operating expenses	[2]	317		292		983		865	[3]
Group Reporting Basis Profit (Loss) Before Income Tax Expense		(29)		(7)		(96)		234
Group Reporting Basis Total assets		18,977		18,968		18,977		18,968
Group Reporting Basis Total loans, net		16,814	[4]	16,781		16,814	[4]	16,781
Group Reporting Basis Goodwill		581		581		581		581
Group Reporting Basis Total deposits		32,051	[4]	34,275		32,051	[4]	34,275
CMB [Member] | Operating Segments [Member]
Operating Statistics [Line Items]
Group Reporting Basis Net interest income	[1]	206		187		587		548
Group Reporting Basis Other operating income		56		55		169	[2]	159	[2]
Group Reporting Basis Total operating income		262		242		756		707
Group Reporting Basis Expected credit losses		(4)		(8)		(50)		(49)
Group Reporting Basis Revenues		266		250		806		756
Group Reporting Basis Operating expenses	[2]	150		142		440		422	[3]
Group Reporting Basis Profit (Loss) Before Income Tax Expense		116		108		366		334	[8]
Group Reporting Basis Total assets		25,289		24,154		25,289		24,154
Group Reporting Basis Total loans, net		24,126	[4]	22,959		24,126	[4]	22,959
Group Reporting Basis Goodwill		358		358		358		358
Group Reporting Basis Total deposits		22,440	[4]	23,919		22,440	[4]	23,919
GBM [Member] | Operating Segments [Member]
Operating Statistics [Line Items]
Group Reporting Basis Net interest income	[1]	157		131		457		439
Group Reporting Basis Other operating income		195		125		644	[2]	436	[2]
Group Reporting Basis Total operating income		352		256		1,101		875
Group Reporting Basis Expected credit losses		(30)		(18)		(187)		(55)
Group Reporting Basis Revenues		382		274		1,288		930
Group Reporting Basis Operating expenses	[2]	203		214		628		686	[3]
Group Reporting Basis Profit (Loss) Before Income Tax Expense		179		60		660		244	[8]
Group Reporting Basis Total assets		76,194		90,779		76,194		90,779
Group Reporting Basis Total loans, net		17,464	[4]	18,843		17,464	[4]	18,843
Group Reporting Basis Goodwill		0		0		0		0
Group Reporting Basis Total deposits		29,777	[4]	21,628		29,777	[4]	21,628
PB [Member] | Operating Segments [Member]
Operating Statistics [Line Items]
Group Reporting Basis Net interest income	[1]	42		56		132		165
Group Reporting Basis Other operating income		17		31		53	[2]	73	[2]
Group Reporting Basis Total operating income		59		87		185		238
Group Reporting Basis Expected credit losses		(1)		(2)		(4)		1
Group Reporting Basis Revenues		60		89		189		237
Group Reporting Basis Operating expenses	[2]	59		67		181		191	[3]
Group Reporting Basis Profit (Loss) Before Income Tax Expense		1		22		8		46
Group Reporting Basis Total assets		6,742		7,684		6,742		7,684
Group Reporting Basis Total loans, net		5,656	[4]	5,900		5,656	[4]	5,900
Group Reporting Basis Goodwill		321		321		321		321
Group Reporting Basis Total deposits		8,167	[4]	9,737		8,167	[4]	9,737
CC [Member] | Operating Segments [Member]
Operating Statistics [Line Items]
Group Reporting Basis Net interest income	[1]	17		(10)		50		(9)
Group Reporting Basis Other operating income		70		33		225	[2]	205	[2]
Group Reporting Basis Total operating income		87		23		275		196
Group Reporting Basis Expected credit losses		(1)		1		3		0
Group Reporting Basis Revenues		88		22		272		196
Group Reporting Basis Operating expenses	[2]	55		103		641		338	[3]
Group Reporting Basis Profit (Loss) Before Income Tax Expense		33		(81)		(369)		(142)
Group Reporting Basis Total assets		78,421		90,262		78,421		90,262
Group Reporting Basis Total loans, net		1,306	[4]	3,751		1,306	[4]	3,751
Group Reporting Basis Goodwill		0		0		0		0
Group Reporting Basis Total deposits		5,211	[4]	8,394		5,211	[4]	8,394
Restatement Adjustment [Member] | RBWM [Member] | Operating Segments [Member]
Operating Statistics [Line Items]
Group Reporting Basis Other operating income				11				35
Group Reporting Basis Operating expenses				11				35
Restatement Adjustment [Member] | GBM [Member] | Operating Segments [Member]
Operating Statistics [Line Items]
Group Reporting Basis Other operating income				14				51
Group Reporting Basis Operating expenses				$ 14				$ 51
Group Reporting Basis Total loans, net		200				200
Group Reporting Basis Total deposits		8,600				8,600
Restatement Adjustment [Member] | CC [Member] | Operating Segments [Member]
Operating Statistics [Line Items]
Group Reporting Basis Total loans, net		2,000				2,000
Group Reporting Basis Total deposits		$ 1,500				$ 1,500

[1] Net interest income of each segment represents the difference between actual interest earned on assets and interest incurred on liabilities of the segment adjusted for a funding charge or credit. Segments are charged a cost to fund assets (e.g., customer loans) and receive a funding credit for funds provided (e.g., customer deposits) based on equivalent market rates. The objective of these charges/credits is to transfer interest rate risk from the segments to one centralized unit in Balance Sheet Management and more appropriately reflect the profitability of the segments.

[2] Expenses for the segments include fully apportioned corporate overhead expenses.

[3] During the fourth quarter of 2017, we changed our presentation for certain cost reimbursements that were previously netted as an offset to affiliate expense and began presenting these reimbursements gross in affiliate income. As a result, we have reclassified prior period amounts in order to conform to the current year presentation, which increased both RBWM other operating income and RBWM operating expenses $11 million and $35 million and also increased both GB&M other operating income and GB&M operating expenses $14 million and $51 million during the three and nine months ended September 30, 2017, respectively. See Note 14, "Related Party Transactions," for additional information.

[4] In addition to the changes discussed above, in conjunction with HSBC's adoption of the requirements of IFRS 9 we also adopted changes in presentation under the Group Reporting Basis related to affiliate loans and deposits as well as cash collateral posted and received. Beginning January 1, 2018, affiliate loans have been reclassified from other assets to loans, affiliate deposits have been reclassified from other liabilities to deposits, cash collateral posted has been reclassified from loans to other assets and cash collateral received has been reclassified from deposits to other liabilities. As a result of these changes, total loans, net and total deposits in the GB&M segment increased $0.2 billion and $8.6 billion, respectively, and total loans, net and total deposits in the CC segment decreased $2.0 billion and $1.5 billion, respectively, at September 30, 2018.

[5] See Note 15, "Business Segments," for a reconciliation of total other revenues on a U.S. GAAP basis to other operating income for each business segment under the Group Reporting Basis.

[6] The following table summarizes assets and liabilities related to our consolidated variable interest entities ("VIEs") at September 30, 2018 and December 31, 2017 which are consolidated on our balance sheet. Assets and liabilities exclude intercompany balances that eliminate in consolidation. See Note 17, "Variable Interest Entities," for additional information. Assets: Other assets $121 million and $154 million. Total assets $121 million and $154 million respectively. Liabilities: Long-term debt $73 million and $73 million, Interest, taxes and other liabilities $61 million and $59 million. Total liabilities $134 million and $132 million, respectively.

[7] Represents differences in financial statement presentation between U.S. GAAP and the Group Reporting Basis.

[8] Represents adjustments associated with differences between U.S. GAAP and the Group Reporting Basis.