Pension and Other Postretirement Benefits-Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Postretirement Plans Other Than Pensions [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost (less than $1 million for all periods for net periodic postretirement benefit cost)	$ 1	$ 1
HSBC North America [Member] | HSBC North America Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	15	15
Expected return on plan assets	(18)	(21)
Amortization of net actuarial (gain) loss	7	5
Administrative costs	1	1
Net periodic benefit cost (less than $1 million for all periods for net periodic postretirement benefit cost)	$ 5	$ 0