Pension and Other Postretirement Benefits-Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Postretirement Plans Other Than Pensions [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	$ 1	$ 0	$ 1	$ 1
Net periodic benefit cost	1	0	1	1
HSBC North America [Member] | HSBC North America Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	15	18	30	36
Expected return on plan assets	(19)	(22)	(40)	(43)
Amortization of net actuarial loss	5	8	10	18
Administrative costs	1	1	2	2
Net periodic benefit cost	$ 2	$ 5	$ 2	$ 13