Pension and Other Postretirement Benefits-Net Periodic Benefit Cost (Detail) - HSBC North America [Member] - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
HSBC North America Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost on projected benefit obligation	$ 11	$ 11	$ 32	$ 35
Expected return on plan assets	(15)	(14)	(44)	(53)
Amortization of net actuarial loss	0	0	0	4
Administrative costs	1	1	3	3
Pension (income) expense	(3)	(2)	(9)	(11)
Postretirement Plans Other Than Pensions [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension (income) expense	$ 1	$ 1	$ 1	$ 1