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Price Risk Management Net realized and unrealized gains and losses on derivative transactions (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended 9 Months Ended
Sep. 30, 2014
Sep. 30, 2013
Sep. 30, 2014
Sep. 30, 2013
Commodity contracts:        
Electricity $ 8 $ (1) $ (21) $ 17
Natural Gas $ 25 $ 10 $ (17) $ 30