Derivative Financial Instruments - Additional Information (Detail) (USD $)	9 Months Ended	12 Months Ended
	Sep. 30, 2012	Dec. 31, 2011
Derivative [Line Items]
Collateral held by third party swap	$ 7,000,000	$ 10,000,000
Fair value of our agreements with affiliate counterparties required the affiliate to provide collateral	183,000,000	584,000,000
Notional values of derivative contracts	27,627,000,000	40,854,000,000
Increase in debt carrying value due to fair value hedge	8,000,000	36,000,000
Gains and losses on current derivative instruments designated as cash flow hedges are reported in OCI net of tax and totaled a loss	377,000,000	448,000,000
Unrealized net losses on cash flow hedges	220,000,000
Unrealized net losses on cash flow hedges after tax	142,000,000
Termination of non-qualifying hedge	4,200,000,000
HSBC Bank USA [Member]
Derivative [Line Items]
Notional values of derivative contracts	$ 27,500,000,000	$ 40,400,000,000