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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, unless otherwise specified
 12 Months Ended 48 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2015
Gain (Loss) on Derivatives [Member]
          
Commodity contracts: [Abstract]          
Commodity Contracts, Electricity $ 8 $ 21 $ 42 $ 64 $ 135
Commodity Contracts, Natural Gas 6 24 72 132 234
Net unrealized loss $ 14 $ 45 $ 114 $ 196 $ 369