Guarantee Arrangements, Pledged Assets and Repurchase Agreements - Additional Information (Detail) - USD ($) $ in Millions		12 Months Ended				36 Months Ended
		Dec. 31, 2018		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2007
Guarantor Obligations [Line Items]
Notional/maximum exposure to loss		$ 54,356	[1]	$ 51,036
Deferred fees		2		1
Carrying value of swap		5,556,668		4,793,784
Net rental expense		103		94	$ 80
Pledged assets not separately reported, trading assets	[2]	2,465		3,185
Collateral accepted under security resale agreements, fair value		13,004		34,759
Collateral accepted under security repurchase agreements but not reported on the balance sheet that can be sold or repledged, fair value		11,904		32,459
Collateral accepted under security repurchase agreements that has been sold or repledged under repurchase agreements or to cover short sales		243		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		822		524
Trading assets [Member]
Guarantor Obligations [Line Items]
Pledged assets not separately reported, trading assets		2,464		3,185
Financial standby letters of credit, net of participations [Member]
Guarantor Obligations [Line Items]
Notional/maximum exposure to loss	[3],[4]	5,302		5,128
Performance (non-financial) guarantees, net of participations [Member]
Guarantor Obligations [Line Items]
Notional/maximum exposure to loss	[3],[4]	3,670		3,580
Obligation to Repurchase Receivables Sold [Member]
Guarantor Obligations [Line Items]
Outstanding repurchase demands		2		3	6
Other liabilities [Member]
Guarantor Obligations [Line Items]
Allowance for credit losses on unfunded standby letters of credit		16		26
Other liabilities [Member] | Standby Letters of Credit [Member]
Guarantor Obligations [Line Items]
Deferred fees		43		48
Swap [Member]
Guarantor Obligations [Line Items]
Carrying value of swap		40		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	$ 71

[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] Trading assets are primarily pledged against liabilities associated with repurchase agreements.

[3] 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.

[4] Includes $1,321 million and $1,264 million of both financial and performance standby letters of credit issued for the benefit of HSBC affiliates at December 31, 2018 and 2017, respectively.