Pension and Other Postretirement Benefits-Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Postretirement Plans Other Than Pensions [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	$ 0	$ 0	$ 1	$ 1
Amortization of net actuarial (gain) loss	0	0	(1)	0
Net periodic benefit cost	0	0	0	1
HSBC North America [Member] | HSBC North America Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	14	16	43	46
Expected return on plan assets	(20)	(20)	(57)	(60)
Amortization of net actuarial (gain) loss	4	5	16	15
Administrative costs	1	1	3	3
Net periodic benefit cost	$ (1)	$ 2	$ 5	$ 4