Pension and Other Postretirement Benefits-Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Jun. 30, 2020	Sep. 30, 2020	Sep. 30, 2019
Defined Benefit Plan Disclosure [Line Items]
Expected long-term rate of return on Plan assets	3.50%		5.00%
HSBC North America [Member] | Postretirement Plans Other Than Pensions [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost on projected benefit obligation	$ 0	$ 0		$ 1	$ 1
Amortization of net actuarial loss	0	0		(1)	(1)
Net periodic benefit cost	0	0		0	0
HSBC North America [Member] | HSBC North America Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost on projected benefit obligation	11	14		35	43
Expected return on plan assets	(14)	(20)		(53)	(57)
Amortization of net actuarial loss	0	4		4	16
Administrative costs	1	1		3	3
Net periodic benefit cost	$ (2)	$ (1)		$ (11)	$ 5