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Derivative Instruments (Details 4) (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 6 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Jun. 30, 2013
Jun. 30, 2012
Derivative Instruments, Gain (Loss) [Line Items]        
Gains (Loss) Recognized in OCI (Effective Portion) $ 48 $ (48) $ 61 $ (48)
Gain (Loss) Reclassified from AOCI into Income (EffectivePortion) (10) [1] 4 [1] (19) [1] 14 [1]
Interest Rate Swap [Member]
        
Derivative Instruments, Gain (Loss) [Line Items]        
Gains (Loss) Recognized in OCI (Effective Portion) 48 [2] (35) [2] 61 [2] (34) [2]
Gain (Loss) Reclassified from AOCI into Income (EffectivePortion) (10) [1],[2] (8) [1],[2] (19) [1],[2] (15) [1],[2]
Power Derivative Instruments [Member] | Commodity Option [Member]
        
Derivative Instruments, Gain (Loss) [Line Items]        
Gains (Loss) Recognized in OCI (Effective Portion) 0 [3] (29) [3] 0 [3] (44) [3]
Gain (Loss) Reclassified from AOCI into Income (EffectivePortion) 0 [1],[3] 29 [1],[3] 0 [1],[3] 67 [1],[3]
Natural Gas Derivative Instruments [Member] | Commodity Option [Member]
        
Derivative Instruments, Gain (Loss) [Line Items]        
Gains (Loss) Recognized in OCI (Effective Portion) 0 [3] 16 [3] 0 [3] 30 [3]
Gain (Loss) Reclassified from AOCI into Income (EffectivePortion) $ 0 [1],[3] $ (17) [1],[3] $ 0 [1],[3] $ (38) [1],[3]
[1] Cumulative cash flow hedge losses, net of tax, remaining in AOCI were $178 million and $242 million at June 30, 2013 and December 31, 2012, respectively.
[2] We did not record any gain (loss) on hedge ineffectiveness related to our interest rate swaps designated as cash flow hedges during the three and six months ended June 30, 2013 and 2012.
[3] There were no commodity derivative instruments designated as cash flow hedges during the three and six months ended June 30, 2013. We recorded a gain on hedge ineffectiveness of nil and $2 million related to our commodity derivative instruments designated as cash flow hedges during the three and six months ended June 30, 2012.