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Financial Instruments, Derivatives, Guarantees and Concentration of Credit Risk (Details 5) (Fair Value, Concentration of Risk, Maximum Amount of Loss, USD $)
In Millions
Sep. 30, 2011
Counterparty type 
Credit risk exposure before credit reserve$ 499
Credit reserves0
Credit exposure after credit reserve499
Investment Grade | Gas and electric utilities
 
Counterparty type 
Credit risk exposure before credit reserve3[1]
Investment Grade | Energy marketers and traders
 
Counterparty type 
Credit risk exposure before credit reserve0[1]
Investment Grade | Financial institutions
 
Counterparty type 
Credit risk exposure before credit reserve422[1]
Investment Grade
 
Counterparty type 
Credit risk exposure before credit reserve425[1]
Gas and electric utilities
 
Counterparty type 
Credit risk exposure before credit reserve3
Energy marketers and traders
 
Counterparty type 
Credit risk exposure before credit reserve74
Financial institutions
 
Counterparty type 
Credit risk exposure before credit reserve$ 422
[1]We determine investment grade primarily using publicly available credit ratings. We include counterparties with a minimum Standard & Poor’s rating of BBB- or Moody’s Investors Service rating of Baa3 in investment grade.