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Derivative Instruments (Details 4) (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 9 Months Ended
Sep. 30, 2013
Sep. 30, 2012
Sep. 30, 2013
Sep. 30, 2012
Derivative Instruments, Gain (Loss) [Line Items]        
Gains (Loss) Recognized in OCI (Effective Portion) $ 12 [1] $ (23) [1] $ 73 [1] $ (71) [1]
Gain (Loss) Reclassified from AOCI into Income (EffectivePortion) (19) [1],[2] 1 [1],[2] (38) [1],[2] 15 [1],[2]
Interest Rate Swap [Member]
        
Derivative Instruments, Gain (Loss) [Line Items]        
Gains (Loss) Recognized in OCI (Effective Portion) 12 [3] (14) [3] 73 [3] (48) [3]
Gain (Loss) Reclassified from AOCI into Income (EffectivePortion) (19) [2],[3],[4] (8) [2],[3] (38) [2],[3],[4] (23) [2],[3]
Power Derivative Instruments [Member] | Commodity Option [Member]
        
Derivative Instruments, Gain (Loss) [Line Items]        
Gains (Loss) Recognized in OCI (Effective Portion) 0 [5] (24) [5] 0 [5] (68) [5]
Gain (Loss) Reclassified from AOCI into Income (EffectivePortion) 0 [2],[5] 24 [2],[5] 0 [2],[5] 91 [2],[5]
Natural Gas Derivative Instruments [Member] | Commodity Option [Member]
        
Derivative Instruments, Gain (Loss) [Line Items]        
Gains (Loss) Recognized in OCI (Effective Portion) 0 [5] 15 [5] 0 [5] 45 [5]
Gain (Loss) Reclassified from AOCI into Income (EffectivePortion) $ 0 [2],[5] $ (15) [2],[5] $ 0 [2],[5] $ (53) [2],[5]
[1] We recorded income tax expense of $7 million and $4 million for the three and nine months ended September 30, 2013, respectively, and income tax benefits of $3 million and $7 million for the three and nine months ended September 30, 2012, respectively, in AOCI related to our cash flow hedging activities.
[2] Cumulative cash flow hedge losses, net of tax, remaining in AOCI were $173 million and $242 million at September 30, 2013 and December 31, 2012, respectively.
[3] 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 nine months ended September 30, 2013 and 2012.
[4] Includes a loss of $7 million that was reclassified from AOCI to interest expense for each of the three and nine months ended September 30, 2013 where the hedged transactions are no longer expected to occur.
[5] There were no commodity derivative instruments designated as cash flow hedges during the three and nine months ended September 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 nine months ended September 30, 2012.