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Fair Value of Derivative and Other Financial Instruments - Statement of Earnings Information (Details) - PNM - Designated as Hedging Instrument - Commodity derivatives - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
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Sep. 30, 2017 | Sep. 30, 2016 | Sep. 30, 2017 | Sep. 30, 2016
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Total gain (loss) | $ (2,251) | $ 1,651 | $ 408 | $ (899)
Electric operating revenues |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Total gain (loss) | (2,237) | 1,652 | 5,697 | 214
Cost of energy |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Total gain (loss) | $ (14) | $ (1) | $ (5,289) | $ (1,113)

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