Pension and Other Postretirement Benefits-Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Postretirement Plans Other Than Pensions [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	$ 0	$ 1
Net periodic benefit cost	0	1
Hsbc North America [Member] | HSBC North America Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	15	18
Expected return on plan assets	(21)	(21)
Amortization of net actuarial (gain) loss	5	10
Administrative costs	1	1
Net periodic benefit cost	$ 0	$ 8