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Derivative Financial Instruments (Narrative) (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Dec. 31, 2015
Derivative Financial Instruments [Abstract]          
Accumulated other comprehensive loss $ (54)   $ (54)   $ (1,165)
Energy trading contracts, gain (loss) $ 2,700 $ 2,900 $ 5,700 $ 10,700