Business Segments - Summary on Reconciliation of Results Under Ifrs to Us Gaap (Detail) - USD ($) $ in Millions		3 Months Ended										12 Months Ended
		Dec. 31, 2018		Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017		Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Operating Statistics [Line Items]
Net Interest Income												$ 2,559		$ 2,387		$ 2,471
Group Reporting Basis Other operating income	[1]											1,604		1,727		1,595
Group Reporting Basis Total operating income												4,163		4,114		4,066
Expected credit losses/provision for credit losses												(199)		(119)		494
Group Reporting Basis Revenues												4,362		4,233		3,572
Operating Expenses	[1]											3,633		3,333	[2]	3,278	[2]
Group Reporting Basis Profit (Loss) Before Income Tax Expense												729		900		294
Total Assets		$ 203,375					$ 220,313					203,375		220,313		246,506
Group Reporting Basis Total loans, net		67,040	[3]				68,215					67,040	[3]	68,215		72,464
Group Reporting Basis Goodwill		1,260					1,260					1,260		1,260		1,264
Group Reporting Basis Total deposits		97,371	[3]				97,294					97,371	[3]	97,294		95,344
Net interest income		586		$ 559	$ 555	$ 552	534		$ 560	$ 582	$ 597	2,252		2,273		2,484
Other operating income		363		486	537	513	458		393	558	593	1,899	[1]	2,002	[1]	1,404	[1],[4]
Total operating income												4,151		4,275		3,888
Provision for credit losses		23		20	(45)	(71)	(45)		(22)	(21)	(77)	(73)		(165)		372
Total income												4,224		4,440		3,516
Operating expenses		780		790	786	1,282	903		820	849	819	3,638	[1]	3,391	[1],[2]	3,298	[1],[2]
Income before income tax		146		$ 235	$ 351	$ (146)	134		$ 155	$ 312	$ 448	586		1,049		218
Total assets		172,448	[5]				187,235	[5]				172,448	[5]	187,235	[5]	201,301
Total loans, net		68,437	[3]				71,882					68,437	[3]	71,882		72,858
Goodwill		1,607					1,607					1,607		1,607		1,612
Total deposits		110,955	[3]				118,702					110,955	[3]	118,702		129,248
Group Reporting Basis Adjustments [Member]
Operating Statistics [Line Items]
Net Interest Income												(16)		(48)		(76)
Group Reporting Basis Other operating income	[1]											(9)		200		(104)
Group Reporting Basis Total operating income												(25)		152		(180)
Expected credit losses/provision for credit losses												115		(72)		(78)
Group Reporting Basis Revenues												(140)		224		(102)
Operating Expenses	[1]											3		75	[2]	(26)	[2]
Group Reporting Basis Profit (Loss) Before Income Tax Expense												(143)		149		(76)
Total Assets		(30,927)					(33,078)					(30,927)		(33,078)		(45,234)
Group Reporting Basis Total loans, net		(2,428)	[3]				(1,397)					(2,428)	[3]	(1,397)		(405)
Group Reporting Basis Goodwill		347					347					347		347		348
Group Reporting Basis Total deposits		(2,017)	[3]				(3,261)					(2,017)	[3]	(3,261)		(4,379)
Group Reporting Basis Reclassifications [Member]
Operating Statistics [Line Items]
Net Interest Income												(291)		(66)		89
Group Reporting Basis Other operating income	[1]											304		75		(87)
Group Reporting Basis Total operating income												13		9		2
Expected credit losses/provision for credit losses												11		26		(44)
Group Reporting Basis Revenues												2		(17)		46
Operating Expenses	[1]											2		(17)	[2]	46	[2]
Group Reporting Basis Profit (Loss) Before Income Tax Expense												0		0		0
Total Assets		0					0					0		0		29
Group Reporting Basis Total loans, net		3,825	[3]				5,064					3,825	[3]	5,064		799
Group Reporting Basis Goodwill		0					0					0		0		0
Group Reporting Basis Total deposits		15,601	[3]				24,669					15,601	[3]	24,669		38,283
Reclassification [Member]
Operating Statistics [Line Items]
Net Interest Income												0		0		(2)
Group Reporting Basis Other operating income	[1]											0		0		2
Group Reporting Basis Total operating income												0		0		0
Expected credit losses/provision for credit losses												0		0		0
Group Reporting Basis Revenues												0		0		0
Operating Expenses	[1]											0		0	[2]	0	[2]
Group Reporting Basis Profit (Loss) Before Income Tax Expense												0		0		0
Total Assets		0					0					0		0		0
Group Reporting Basis Total loans, net		0	[3]				0					0	[3]	0		0
Group Reporting Basis Goodwill		0					0					0		0		0
Group Reporting Basis Total deposits		0	[3]				0					0	[3]	0		0
RBWM [Member] | Operating Segments [Member]
Operating Statistics [Line Items]
Net Interest Income												889		886		818
Group Reporting Basis Other operating income	[1]											297		526		363
Group Reporting Basis Total operating income												1,186		1,412		1,181
Expected credit losses/provision for credit losses												38		25		70
Group Reporting Basis Revenues												1,148		1,387		1,111
Operating Expenses	[1]											1,286		1,185	[2]	1,131	[2]
Group Reporting Basis Profit (Loss) Before Income Tax Expense												(138)		202		(20)
Total Assets		19,000					18,707					19,000		18,707		19,618
Group Reporting Basis Total loans, net		16,918	[3]				16,685					16,918	[3]	16,685		16,889
Group Reporting Basis Goodwill		581					581					581		581		581
Group Reporting Basis Total deposits		32,612	[3]				34,186					32,612	[3]	34,186		32,472
CMB [Member] | Operating Segments [Member]
Operating Statistics [Line Items]
Net Interest Income												797		739		749
Group Reporting Basis Other operating income	[1]											226		216		229
Group Reporting Basis Total operating income												1,023		955		978
Expected credit losses/provision for credit losses												(43)		(52)		50
Group Reporting Basis Revenues												1,066		1,007		928
Operating Expenses	[1]											589		558	[2]	591	[2]
Group Reporting Basis Profit (Loss) Before Income Tax Expense												477		449		337
Total Assets		25,047					23,644					25,047		23,644		25,082
Group Reporting Basis Total loans, net		23,913	[3]				22,444					23,913	[3]	22,444		24,125
Group Reporting Basis Goodwill		358					358					358		358		358
Group Reporting Basis Total deposits		23,604	[3]				24,239					23,604	[3]	24,239		22,005
GBM [Member] | Operating Segments [Member]
Operating Statistics [Line Items]
Net Interest Income												610		585		567
Group Reporting Basis Other operating income	[1]											751		617		785
Group Reporting Basis Total operating income												1,361		1,202		1,352
Expected credit losses/provision for credit losses												(194)		(94)		383
Group Reporting Basis Revenues												1,555		1,296		969
Operating Expenses	[1]											834		881	[2]	986	[2]
Group Reporting Basis Profit (Loss) Before Income Tax Expense												721		415		(17)
Total Assets		73,187					72,734					73,187		72,734		85,215
Group Reporting Basis Total loans, net		18,895	[3]				19,125					18,895	[3]	19,125		21,914
Group Reporting Basis Goodwill		0					0					0		0		0
Group Reporting Basis Total deposits		30,181	[3]				25,476					30,181	[3]	25,476		22,260
PB [Member] | Operating Segments [Member]
Operating Statistics [Line Items]
Net Interest Income												172		217		207
Group Reporting Basis Other operating income	[1]											73		94		89
Group Reporting Basis Total operating income												245		311		296
Expected credit losses/provision for credit losses												(4)		2		0
Group Reporting Basis Revenues												249		309		296
Operating Expenses	[1]											236		244	[2]	232	[2]
Group Reporting Basis Profit (Loss) Before Income Tax Expense												13		65		64
Total Assets		6,797					8,190					6,797		8,190		7,714
Group Reporting Basis Total loans, net		5,715	[3]				6,405					5,715	[3]	6,405		6,024
Group Reporting Basis Goodwill		321					321					321		321		325
Group Reporting Basis Total deposits		8,193	[3]				8,470					8,193	[3]	8,470		11,618
CC [Member] | Operating Segments [Member]
Operating Statistics [Line Items]
Net Interest Income												91		(40)		132
Group Reporting Basis Other operating income	[1]											257		274		127
Group Reporting Basis Total operating income												348		234		259
Expected credit losses/provision for credit losses												4		0		(9)
Group Reporting Basis Revenues												344		234		268
Operating Expenses	[1]											688		465	[2]	338	[2]
Group Reporting Basis Profit (Loss) Before Income Tax Expense												(344)		(231)		(70)
Total Assets		79,344					97,038					79,344		97,038		108,877
Group Reporting Basis Total loans, net		1,599	[3]				3,556					1,599	[3]	3,556		3,512
Group Reporting Basis Goodwill		0					0					0		0		0
Group Reporting Basis Total deposits		2,781	[3]				$ 4,923					2,781	[3]	$ 4,923		$ 6,989
Out of Period Adjustments [Member] | GBM [Member] | Operating Segments [Member]
Operating Statistics [Line Items]
Group Reporting Basis Total loans, net		(100)										(100)
Group Reporting Basis Total deposits		8,300										8,300
Out of Period Adjustments [Member] | CC [Member] | Operating Segments [Member]
Operating Statistics [Line Items]
Group Reporting Basis Total loans, net		(2,200)										(2,200)
Group Reporting Basis Total deposits		$ 300										$ 300

[1] Represents adjustments associated with differences between U.S. GAAP and the Group Reporting Basis. These adjustments, which are more fully described above, consist of the following: NetInterestIncome OtherRevenues Provisionfor CreditLosses OperatingExpenses Profit (Loss)before IncomeTax Expense TotalAssets (in millions)December 31, 2018 Deposit incentives$9 $— $— $20 $(11) $(10)Derivatives— — — — — (30,866)Expected credit losses / loan impairment(6) — 118 — (124) (386)Goodwill— — — — — 347Loan origination(29) (3) — (14) (18) 45Loans held for sale9 6 (3) — 18 45Pension and other postretirement benefit costs— — — 8 (8) (44)Property— — — (8) 8 7Securities transferred to held-to-maturity— — — — — (76)Other1 (12) — (3) (8) 11Total adjustments$(16) $(9) $115 $3 $(143) $(30,927) December 31, 2017 Derivatives$— $— $— $— $— $(32,900)Expected credit losses / loan impairment(43) — (72) — 29 (332)Goodwill— (1) — — (1) 347Loan origination(19) — — (14) (5) 42Loans held for sale9 141 — — 150 21Pension and other postretirement benefit costs— — — 94 (94) (263)Property— — — (11) 11 8Structured notes and deposits— 85 — — 85 —Other5 (25) — 6 (26) (1)Total adjustments$(48) $200 $(72) $75 $149 $(33,078) December 31, 2016 Derivatives$— $— $— $— $— $(44,911)Expected credit losses / loan impairment(57) 7 (58) 1 7 (326)Goodwill— — — — — 348Loan origination(21) — — (19) (2) 35Loans held for sale— (80) (20) — (60) (31)Pension and other postretirement benefit costs— — — 19 (19) (191)Property— — — (19) 19 16Unquoted equity securities— — — — — (163)Other2 (31) — (8) (21) (11)Total adjustments$(76) $(104) $(78) $(26) $(76) $(45,234)

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

[3] In addition to the changes discussed above, in conjunction with HSBC's adoption of the requirements of IFRS 9 HUSI 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 decreased $0.1 billion and increased $8.3 billion, respectively, and total loans, net and total deposits in the CC segment decreased $2.2 billion and increased $0.3 billion, respectively, at December 31, 2018.

[4] See Note 23, "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.

[5] The following table summarizes assets and liabilities related to our consolidated variable interest entities ("VIEs") at December 31, 2018 and December 31, 2017 which are consolidated on our balance sheet. Assets and liabilities exclude intercompany balances that eliminate in consolidation. See Note 25, "Variable Interest Entities," for additional information. Assets: Other assets $112 million and $154 million. Total assets $112 million and $154 million respectively. Liabilities: Long-term debt $66 million and $73 million, Interest, taxes and other liabilities $37 million and $59 million. Total liabilities $103 million and $132 million, respectively.