Guarantee Arrangements, Pledged Assets and Repurchase Agreements - Additional Information (Detail) - USD ($) $ in Millions		3 Months Ended	36 Months Ended
		Mar. 31, 2017	Dec. 31, 2007	Mar. 31, 2018	Dec. 31, 2017
Guarantor Obligations [Line Items]
Notional/maximum exposure to loss				$ 46,595	$ 51,036
Carrying value of swap				5,609,194	4,793,784
Carrying value				(234)	(303)
Collateral accepted under security resale agreements, fair value				12,749	34,759
Collateral accepted under security repurchase agreements but not reported on the balance sheet that can be sold or repledged, fair value				10,649	32,459
Collateral accepted under security repurchase agreements that has been sold or repledged under repurchase agreements or to cover short sales				321	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				1,361	524
Trading Securities [Member]
Guarantor Obligations [Line Items]
Securities pledged as collateral that can be sold or repledged by the secured party, fair value				2,934	3,185
Financial standby letters of credit, net of participations [Member]
Guarantor Obligations [Line Items]
Notional/maximum exposure to loss	[1],[2]			5,533	5,128
Carrying value	[1],[2]			0	0
Performance (non-financial) guarantees, net of participations [Member]
Guarantor Obligations [Line Items]
Notional/maximum exposure to loss	[1],[2]			3,465	3,580
Carrying value	[1],[2]			0	0
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				4,000	4,100
Interest taxes and other liabilities [Member]
Guarantor Obligations [Line Items]
Obligation to return securities received as collateral				500
Other liabilities [Member]
Guarantor Obligations [Line Items]
Allowance for credit losses on unfunded standby letters of credit				26	26
Other liabilities [Member] | Standby Letters of Credit [Member]
Guarantor Obligations [Line Items]
Deferred fees on standby letters of credit				44	48
Other assets [Member]
Guarantor Obligations [Line Items]
Collateral received that can be sold or repledged				500
Swap [Member]
Guarantor Obligations [Line Items]
Carrying value of swap				$ 50	$ 52
Other Income [Member] | Visa Class B Shares [Member]
Guarantor Obligations [Line Items]
Gain on sale of investments		$ 146

[1] 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.

[2] Includes $1,327 million and $1,264 million of both financial and performance standby letters of credit issued for the benefit of HSBC affiliates at March 31, 2018 and December 31, 2017, respectively.