Derivative Instruments- Fair Value of Derivative Contracts Falling under Early Termination Provisions in Net Liability Position (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013
A-
Credit Derivatives [Line Items]
Potential close-out exposures (fair value)	$ 654	[1]
BBB
Credit Derivatives [Line Items]
Potential close-out exposures (fair value)	785	[1]
BB plus
Credit Derivatives [Line Items]
Potential close-out exposures (fair value)	1,523	[1]
A3
Credit Derivatives [Line Items]
Potential close-out exposures (fair value)	654	[1]
Baa2
Credit Derivatives [Line Items]
Potential close-out exposures (fair value)	785	[1]
Ba1
Credit Derivatives [Line Items]
Potential close-out exposures (fair value)	$ 1,523	[1]

[1] The change between rating categories is incremental, not cumulative.