Derivative Financial Instruments - Gains and Losses on Hedged Items in Fair Value Hedges (Detail) - Net Interest Income [Member] - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	[1]	Sep. 30, 2019	[1]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	$ 159	$ (276)	$ (638)		$ (700)
Gain (Loss) on Hedged Items	(172)	242	587		591
Securities available-for-sale [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	182	(327)	(962)		(988)
Gain (Loss) on Hedged Items	(135)	415	1,095		1,253
Deposits [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	33		153
Gain (Loss) on Hedged Items	(69)		(250)
Long-term Debt [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	(56)	51	171		288
Gain (Loss) on Hedged Items	$ 32	$ (173)	$ (258)		$ (662)

[1] During the first nine months of 2020, $5.0 billion of fixed-rate senior debt obligations issued to HSBC North America were recharacterized as time deposits. The cumulative amount of fair value hedging adjustments associated with this debt was reclassified to deposits. See Note 14, "Related Party Transactions," for additional information.