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Fair Value of Derivative and Other Financial Instruments - Effect of Mark-to-Market Instruments on Earnings (Details) - PNMR and PNM - Designated as hedging instrument - Commodity Contract - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Derivative Instruments, Gain (Loss) [Line Items] |  |  |
Gain (loss) | $ (102) | $ (235) | $ (1,261)
Electric operating revenues |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |
Gain (loss) | (50) | 5,151 | (53)
Cost of energy |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |
Gain (loss) | $ (52) | $ (5,386) | $ (1,208)

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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/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4A -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5618551-113959 + Details
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