Pension and Other Postretirement Benefits-Net Periodic Benefit Cost (Detail) - HSBC North America [Member] - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Postretirement Plans Other Than Pensions [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost on projected benefit obligation	$ 1	$ 1	$ 1	$ 1
Amortization of net actuarial loss	(1)	(1)	(1)	(1)
Net periodic benefit cost (less than $1 million for post retirement plans other than pensions)	0	0	0	0
HSBC North America Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost on projected benefit obligation	12	14	24	29
Expected return on plan assets	(20)	(19)	(39)	(37)
Amortization of net actuarial loss	2	5	4	12
Administrative costs	1	1	2	2
Net periodic benefit cost (less than $1 million for post retirement plans other than pensions)	$ (5)	$ 1	$ (9)	$ 6