Guarantee Arrangements, Pledged Assets and Repurchase Agreements - Additional Information (Detail) - USD ($) $ in Millions		36 Months Ended
		Dec. 31, 2007	Sep. 30, 2020	Dec. 31, 2019
Guarantor Obligations [Line Items]
Deferred fees			$ 3	$ 2
Carrying value of swap			3,074,468	4,807,851
Pledged assets not separately reported, trading assets	[1]		3,652	525
Collateral accepted under security resale agreements, fair value			29,716	19,174
Collateral accepted under security repurchase agreements but not reported on the balance sheet that can be sold or repledged, fair value			29,241	18,199
Collateral accepted under security repurchase agreements that has been sold or repledged under repurchase agreements or to cover short sales			917	2,062
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			1,714	0
Trading assets [Member]
Guarantor Obligations [Line Items]
Pledged assets not separately reported, trading assets			3,652	336
Other liabilities [Member]
Guarantor Obligations [Line Items]
Allowance for credit losses on unfunded standby letters of credit			36	26
Other liabilities [Member] | Standby Letters of Credit [Member]
Guarantor Obligations [Line Items]
Deferred fees			44	52
Swap [Member]
Guarantor Obligations [Line Items]
Carrying value of swap			68	85
Mortgage Securitization Activity Related Litigation [Member] | HSBC Securities [Member]
Guarantor Obligations [Line Items]
Mortgage securitization outstanding loan purchase and sold			$ 3,200	$ 3,400

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