Pension and Other Postretirement Benefits-Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Postretirement Plans Other Than Pensions [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	$ 2	$ 2	$ 2
Amortization of net actuarial (gain) loss	(2)	(1)	0
Net periodic benefit cost	0	1	2
HSBC North America [Member] | HSBC North America Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	60	71	70
Expected return on plan assets	(78)	(88)	(86)
Amortization of net actuarial (gain) loss	20	33	42
Administrative costs	6	4	4
Loss on partial settlement of Plan pension obligation	0	35	0
Net periodic benefit cost	8	$ 55	$ 30
Out of Period Adjustments [Member] | HSBC North America [Member] | HSBC North America Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ 10