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Derivative Instruments and Hedging Activities - Summary of Derivative Transactions and Repurchase Agreements Subject to Legally Enforceable Master Netting Arrangements (Detail) (USD $)
In Millions, unless otherwise specified
Jun. 30, 2013
Dec. 31, 2012
Derivatives, Fair Value [Line Items]    
Gross Amounts of Recognized Assets $ 1,144 $ 1,848
Gross Amounts Offset in the Balance Sheet 0 0
Net Amounts of Assets Presented in the Balance Sheet 1,144 1,848
Net Amount 305 468
Gross Amounts of Recognized Liabilities 638 400
Gross Amounts Offset in the Balance Sheet 0 0
Net Amounts of Liabilities Presented in the Balance Sheet 1,533 1,635
Gross Amounts Not Offset in the Balance Sheet 319 922
Net Amount 230 82
Derivatives [Member]
   
Derivatives, Fair Value [Line Items]    
Gross Amounts of Recognized Assets 1,144 1,848
Gross Amounts Offset in the Balance Sheet 0 0
Net Amounts of Assets Presented in the Balance Sheet 1,144 1,848
Net Amount 305 468
Gross Amounts of Recognized Liabilities 638 400
Gross Amounts Offset in the Balance Sheet 0 0
Net Amounts of Liabilities Presented in the Balance Sheet 638 400
Net Amount 230 82
Repurchase Agreements [Member]
   
Derivatives, Fair Value [Line Items]    
Gross Amounts of Recognized Liabilities 895 1,235
Gross Amounts Offset in the Balance Sheet 0 0
Net Amounts of Liabilities Presented in the Balance Sheet 895 1,235
Net Amount 0 0
Financial Instruments [Member]
   
Derivatives, Fair Value [Line Items]    
Gross Amounts Not Offset in the Balance Sheet (252) (220)
Gross Amounts Not Offset in the Balance Sheet (252) (220)
Financial Instruments [Member] | Derivatives [Member]
   
Derivatives, Fair Value [Line Items]    
Gross Amounts Not Offset in the Balance Sheet (252) (220)
Gross Amounts Not Offset in the Balance Sheet (252) (220)
Financial Instruments [Member] | Repurchase Agreements [Member]
   
Derivatives, Fair Value [Line Items]    
Gross Amounts Not Offset in the Balance Sheet 0 0
Collateral Received [Member]
   
Derivatives, Fair Value [Line Items]    
Gross Amounts Not Offset in the Balance Sheet (587) (1,160)
Collateral Received [Member] | Derivatives [Member]
   
Derivatives, Fair Value [Line Items]    
Gross Amounts Not Offset in the Balance Sheet (587) (1,160)
Collateral Pledged [Member]
   
Derivatives, Fair Value [Line Items]    
Gross Amounts Not Offset in the Balance Sheet (1,051) (1,333)
Collateral Pledged [Member] | Derivatives [Member]
   
Derivatives, Fair Value [Line Items]    
Gross Amounts Not Offset in the Balance Sheet (156) (98)
Collateral Pledged [Member] | Repurchase Agreements [Member]
   
Derivatives, Fair Value [Line Items]    
Gross Amounts Not Offset in the Balance Sheet $ (895) $ (1,235)