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Fair Value of Derivative and Other Financial Instruments - Margin, Notional Amounts and Credit Rating (Details) - PNM | 3 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Mar. 31, 2018 USD ($) MWh MMBTU | Dec. 31, 2016 MWh MMBTU | Dec. 31, 2017 USD ($)
Derivative [Line Items] |  |  |
Net exposure | $ | $ 0 |  | $ 0
Commodity derivatives | Fair value hedging | Buy |  |  |
Derivative [Line Items] |  |  |
Economic Hedges (in mmbtu and mwh) | MMBTU | 405,000 | 100,000 |
Commodity derivatives | Fair value hedging | Sell |  |  |
Derivative [Line Items] |  |  |
Economic Hedges (in mmbtu and mwh) | MWh | 40,800 | 0 |

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- Definition The aggregate fair value amounts of derivative instruments that contain credit-risk-related contingent features that are in a net liability position at the end of the reporting period. For nonderivative instruments that are designated and qualify as hedging instruments, the fair value amounts are the carrying value of the nonderivative hedging instrument, including the adjustment for the foreign currency transaction gain (loss) on that instrument. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4H -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624258-113959 + Details
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