Pension and Other Postretirement Benefits-Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
HSBC North America Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost – benefits earned during the period		$ 1.0		$ 1.0	$ 3.0	$ 2.0
Interest cost		16.0		18.0	33.0	36.0
Expected return on plan assets		(23.0)		(21.0)	(46.0)	(42.0)
Amortization of net actuarial loss		10.0		9.0	19.0	19.0
Net periodic benefit cost		4.0		7.0	9.0	15.0
Postretirement Plans Other Than Pensions [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost		0.0		1.0	1.0	1.0
Net periodic benefit cost		$ 0.0		$ 1.0	$ 1.0	$ 1.0
Hsbc North America [Member] | HSBC North America Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Contributions by employer			$ 46.0
Scenario, Forecast [Member]
Defined Benefit Plan Disclosure [Line Items]
Increase to pension expense beginning in Q3, due to change in mortality assumptions	$ 3.5