Pension and Other Postretirement Benefits-Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Postretirement Plans Other Than Pensions [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	$ 1	$ 1	$ 1	$ 1
Amortization of net actuarial (gain) loss	(1)	0	(1)	0
Net periodic benefit cost	0	1	0	1
HSBC North America [Member] | HSBC North America Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	14	15	29	30
Expected return on plan assets	(19)	(19)	(37)	(40)
Amortization of net actuarial (gain) loss	5	5	12	10
Administrative costs	1	1	2	2
Net periodic benefit cost	$ 1	$ 2	$ 6	$ 2