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Fair Value of Derivative and Other Financial Instruments - Statement of Earnings Information (Details) - Designated as Hedging Instrument [Member] - Commodity Contract [Member] - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Derivative Instruments, Gain (Loss) [Line Items]    
Gain (loss) $ 2,539 $ (522)
Electric operating revenues [Member]    
Derivative Instruments, Gain (Loss) [Line Items]    
Gain (loss) 2,684 (472)
Cost of energy [Member]    
Derivative Instruments, Gain (Loss) [Line Items]    
Gain (loss) $ (145) $ (50)