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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 | 12 Months Ended
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Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Derivative Instruments, Gain (Loss) [Line Items] |  |  |
Gain (loss) | $ 6,863 | $ 5,084 | $ 2,836
Electric operating revenues [Member] |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |
Gain (loss) | 7,156 | 4,491 | 1,727
Cost of energy [Member] |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |
Gain (loss) | $ (293) | $ 593 | $ 1,109

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- Definition Amount of increase (decrease) in the fair value of derivatives recognized in the income statement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5618551-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (a),(c),(d),(e) -URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5624171-113959 + Details
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