Pension and Other Postretirement Benefits-Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Postretirement Plans Other Than Pensions [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	$ 0	$ 1	$ 1	$ 2
Net periodic benefit cost	0	1	1	2
Hsbc North America [Member] | HSBC North America Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	18	18	54	53
Expected return on plan assets	(22)	(21)	(65)	(63)
Amortization of net actuarial (gain) loss	7	13	25	32
Administrative costs	1	1	3	4
Net periodic benefit cost	$ 4	$ 11	$ 17	$ 26