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Price Risk Management Future Year Net Unrealized Gain/Loss Recorded at Balance Sheet Date Expected to Become Realized (Details) (Gain (Loss) on Derivatives [Member], USD $)
In Millions
 6 Months Ended 12 Months Ended 42 Months Ended
Dec. 31, 2011
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2014
Gain (Loss) on Derivatives [Member]
            
Commodity contracts:            
Electricity $ 22 $ 0 $ 7 $ 18 $ 39 $ 86
Natural gas 56 1 10 46 97 210
Net unrealized loss $ 78 $ 1 $ 17 $ 64 $ 136 $ 296