Pension and Other Postretirement Benefits-Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Postretirement Plans Other Than Pensions [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost on projected benefit obligation	$ 1	$ 2	$ 2
Amortization of net actuarial loss	0	(2)	(2)
Net periodic benefit cost	1	0	0
HSBC North America [Member] | HSBC North America Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost on projected benefit obligation	45	56	60
Expected return on plan assets	(68)	(78)	(78)
Amortization of net actuarial loss	4	21	31
Administrative costs	4	4	6
Net periodic benefit cost	$ (15)	$ 3	$ 19