Pension and Other Postretirement Benefits-Net Periodic Benefit Cost (Detail) - HSBC North America [Member] - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Postretirement Plans Other Than Pensions [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost (less than $1 million for post retirement plans other than pensions)	$ 1	$ 1
HSBC North America Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost on projected benefit obligation	12	15
Expected return on plan assets	(19)	(18)
Amortization of net actuarial loss	2	7
Administrative costs	1	1
Net periodic benefit cost (less than $1 million for post retirement plans other than pensions)	$ (4)	$ 5