Derivative Instruments - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012	Jun. 30, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Dec. 31, 2010
Derivative [Line Items]
Derivatives designated as cash flow hedges gain (loss), amount expected to be reclassified to future net income (loss), net of tax	$ 2,011	$ 2,087	$ 2,009	$ 1,960	$ 943	$ 924
Year by which all forecasted transactions associated with qualifying cash flow hedges are expected to occur	2045
Derivatives designated as cash flow hedges gain (loss), amount expected to be reclassified to net income (loss) in the next 12 months, net of tax	33
Amount to claim from counterparties if the downgrade provisions had been triggered	22		54
Amount required for disbursement to counterparties if the downgrade provisions had been triggered	5		18
Credit Risk and Counterparty Netting
Derivative [Line Items]
Derivative, net asset position, aggregate fair value	937		1,027
Net fair value derivative liabilities	193		240
Collateral from derivative counterparties	1,010		1,023
Collateral from derivative counterparties, over collateralization	95		50
Collateral to derivative counterparties	24		28
Collateral to derivative counterparties, over collateralization	$ 1		$ 11