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Fair Value Measurements (Details) (USD $)
3 Months Ended9 Months Ended
Sep. 30, 2011
Sep. 30, 2010
Sep. 30, 2011
Sep. 30, 2010
Dec. 31, 2010
Assets     
Nuclear decommissioning trust$ 488,551,000 $ 488,551,000 $ 469,886,000
Changes in fair value for assets and liabilities that are measured at fair value on a recurring basis using Level 3 inputs     
Net derivative balance at beginning of period(47,000,000)(42,000,000)(38,000,000)(10,000,000) 
Total net gains (losses) realized/unrealized:     
Included in earnings1,000,0001,000,0002,000,000(1,000,000) 
Included in other comprehensive income(OCI)2,000,000(11,000,000)1,000,000(20,000,000) 
Deferred as a regulatory asset or liability2,000,000(15,000,000)(4,000,000)(39,000,000) 
Settlements6,000,00012,000,00010,000,00015,000,000 
Transfers into Level 3 from Level 2 (2,000,000)(4,000,000)6,000,000 
Transfers from Level 3 into Level 2(2,000,000)8,000,000(5,000,000)  
Net derivative balance at end of period(38,000,000)(49,000,000)(38,000,000)(49,000,000) 
Net unrealized gains included in earnings related to instruments still held at end of period 1,000,0001,000,000  
Fair value measurement on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1)
     
Assets     
Cash equivalents401,000,000 401,000,000 35,000,000
Nuclear decommissioning trust80,000,000 80,000,000 50,000,000
Total assets481,000,000 481,000,000 85,000,000
Liabilities     
Risk management activities-derivative instruments: Commodity contracts    (1,000,000)
Fair value measurement on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury
     
Assets     
Nuclear decommissioning trust80,000,000 80,000,000 50,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2)
     
Assets     
Risk management activities-derivative instruments: Commodity contracts59,000,000 59,000,000 80,000,000
Nuclear decommissioning trust407,000,000 407,000,000 430,000,000
Total assets466,000,000 466,000,000 510,000,000
Liabilities     
Risk management activities-derivative instruments: Commodity contracts(189,000,000) (189,000,000) (280,000,000)
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Equity securities, U.S. commingled funds
     
Assets     
Nuclear decommissioning trust154,000,000 154,000,000 168,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Cash and cash equivalent funds
     
Assets     
Nuclear decommissioning trust12,000,000 12,000,000 22,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Corporate debt
     
Assets     
Nuclear decommissioning trust64,000,000 64,000,000 60,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Mortgage-backed securities
     
Assets     
Nuclear decommissioning trust76,000,000 76,000,000 81,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Municipality bonds
     
Assets     
Nuclear decommissioning trust81,000,000 81,000,000 79,000,000
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Other.
     
Assets     
Nuclear decommissioning trust20,000,000 20,000,000 20,000,000
Fair value measurement on a recurring basis | Significant Unobservable Inputs (Level 3)
     
Assets     
Risk management activities-derivative instruments: Commodity contracts54,000,000 54,000,000 61,000,000
Total assets54,000,000 54,000,000 61,000,000
Liabilities     
Risk management activities-derivative instruments: Commodity contracts(92,000,000) (92,000,000) (99,000,000)
Fair value measurement on a recurring basis | Other
     
Assets     
Risk management activities-derivative instruments: Commodity contracts(53,000,000) (53,000,000) (28,000,000)
Nuclear decommissioning trust2,000,000 2,000,000 (10,000,000)
Total assets(51,000,000) (51,000,000) (38,000,000)
Liabilities     
Risk management activities-derivative instruments: Commodity contracts162,000,000 162,000,000 256,000,000
Fair value measurement on a recurring basis | Other | Cash and cash equivalent funds
     
Assets     
Nuclear decommissioning trust2,000,000 2,000,000  
Fair value measurement on a recurring basis | Other | Other.
     
Assets     
Nuclear decommissioning trust    (10,000,000)
Fair value measurement on a recurring basis | Fair Value
     
Assets     
Cash equivalents401,000,000 401,000,000 35,000,000
Risk management activities-derivative instruments: Commodity contracts60,000,000 60,000,000 113,000,000
Nuclear decommissioning trust489,000,000 489,000,000 470,000,000
Total assets950,000,000 950,000,000 618,000,000
Liabilities     
Risk management activities-derivative instruments: Commodity contracts(119,000,000) (119,000,000) (124,000,000)
Fair value measurement on a recurring basis | Fair Value | Equity securities, U.S. commingled funds
     
Assets     
Nuclear decommissioning trust154,000,000 154,000,000 168,000,000
Fair value measurement on a recurring basis | Fair Value | U.S. Treasury
     
Assets     
Nuclear decommissioning trust80,000,000 80,000,000 50,000,000
Fair value measurement on a recurring basis | Fair Value | Cash and cash equivalent funds
     
Assets     
Nuclear decommissioning trust14,000,000 14,000,000 22,000,000
Fair value measurement on a recurring basis | Fair Value | Corporate debt
     
Assets     
Nuclear decommissioning trust64,000,000 64,000,000 60,000,000
Fair value measurement on a recurring basis | Fair Value | Mortgage-backed securities
     
Assets     
Nuclear decommissioning trust76,000,000 76,000,000 81,000,000
Fair value measurement on a recurring basis | Fair Value | Municipality bonds
     
Assets     
Nuclear decommissioning trust81,000,000 81,000,000 79,000,000
Fair value measurement on a recurring basis | Fair Value | Other.
     
Assets     
Nuclear decommissioning trust$ 20,000,000 $ 20,000,000 $ 10,000,000