Guarantee Arrangements, Pledged Assets and Repurchase Agreements - Additional Information (Detail) - USD ($) $ in Millions		36 Months Ended
		Dec. 31, 2007	Mar. 31, 2021	Dec. 31, 2020
Guarantor Obligations [Line Items]
Deferred fees			$ 3	$ 3
Carrying value of swap			1,751,830	2,021,133
Pledged assets not separately reported, trading assets	[1]		1,115	1,265
Collateral accepted under security resale agreements, fair value			7,019	41,447
Collateral accepted under security repurchase agreements but not reported on the balance sheet that can be sold or repledged, fair value			6,619	41,047
Collateral accepted under security repurchase agreements that has been sold or repledged under repurchase agreements or to cover short sales			1,046	4,063
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			2,821	2,077
Trading assets [Member]
Guarantor Obligations [Line Items]
Pledged assets not separately reported, trading assets			1,115	1,265
Other liabilities [Member]
Guarantor Obligations [Line Items]
Allowance for credit losses on unfunded standby letters of credit			17	32
Other liabilities [Member] | Standby Letters of Credit [Member]
Guarantor Obligations [Line Items]
Deferred fees			44	44
Swap [Member]
Guarantor Obligations [Line Items]
Carrying value of swap			57	67
Mortgage Securitization Activity Related Litigation [Member] | HSBC Securities [Member]
Guarantor Obligations [Line Items]
Mortgage securitization outstanding loan purchase and sold			$ 3,000	$ 3,100

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