Pension and Other Postretirement Benefits-Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Postretirement Plans Other Than Pensions [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	$ 0	$ 0	$ 1	$ 1
Net periodic benefit cost	0	0	1	1
HSBC North America [Member] | HSBC North America Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	16	18	46	54
Expected return on plan assets	(20)	(22)	(60)	(65)
Amortization of net actuarial loss	5	7	15	25
Administrative costs	1	1	3	3
Net periodic benefit cost	$ 2	$ 4	$ 4	$ 17