Guarantee Arrangements, Pledged Assets and Repurchase Agreements - Additional Information (Detail) - USD ($) $ in Millions		3 Months Ended	9 Months Ended	36 Months Ended
		Jun. 30, 2018	Sep. 30, 2017	Dec. 31, 2007	Sep. 30, 2018		Dec. 31, 2017
Guarantor Obligations [Line Items]
Notional/maximum exposure to loss					$ 56,585	[1]	$ 51,036
Deferred fees					2		1
Carrying value of swap					5,434,475		4,793,784
Collateral accepted under security resale agreements, fair value					8,468		34,759
Collateral accepted under security repurchase agreements but not reported on the balance sheet that can be sold or repledged, fair value					7,368		32,459
Collateral accepted under security repurchase agreements that has been sold or repledged under repurchase agreements or to cover short sales					910		1,231
HSBC Securities [Member]
Guarantor Obligations [Line Items]
Whole loan securitization				$ 24,000
Securities available-for-sale [Member]
Guarantor Obligations [Line Items]
Securities pledged as collateral that can be sold or repledged by the secured party, fair value					1,990		524
Trading assets [Member]
Guarantor Obligations [Line Items]
Securities pledged as collateral that can be sold or repledged by the secured party, fair value					2,664		3,185
Financial standby letters of credit, net of participations [Member]
Guarantor Obligations [Line Items]
Notional/maximum exposure to loss	[2],[3]				5,216		5,128
Performance (non-financial) guarantees, net of participations [Member]
Guarantor Obligations [Line Items]
Notional/maximum exposure to loss	[2],[3]				3,640		3,580
Obligation to Repurchase Receivables Sold [Member]
Guarantor Obligations [Line Items]
Outstanding repurchase demands					2		3
Mortgage Securitization Activity Related Litigation [Member] | HSBC Securities [Member]
Guarantor Obligations [Line Items]
Mortgage securitization outstanding loan purchase and sold					3,800		4,100
Other liabilities [Member]
Guarantor Obligations [Line Items]
Allowance for credit losses on unfunded standby letters of credit					19		26
Other liabilities [Member] | Standby Letters of Credit [Member]
Guarantor Obligations [Line Items]
Deferred fees					44		48
Swap [Member]
Guarantor Obligations [Line Items]
Carrying value of swap					$ 51		$ 52
Visa Class B Shares [Member]
Guarantor Obligations [Line Items]
Loss related to change in conversion rate		$ 7
Other Income (Loss) [Member] | Visa Class B Shares [Member]
Guarantor Obligations [Line Items]
Gain on sale of investments			$ 312

[1] External ratings for most of the obligors are not available. Presented above are the internal credit ratings which are developed using similar methodologies and rating scale equivalent to external credit ratings for purposes of classification as investment grade and non-investment grade.

[2] For standby letters of credit, maximum loss represents losses to be recognized assuming the letters of credit have been fully drawn and the obligors have defaulted with zero recovery.

[3] Includes $1,245 million and $1,264 million of both financial and performance standby letters of credit issued for the benefit of HSBC affiliates at September 30, 2018 and December 31, 2017, respectively.