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Fair Value of Derivative and Other Financial Instruments - Margin, Notional Amounts and Credit Rating (Details) | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2018 USD ($) MMBTU MWh | Dec. 31, 2017 USD ($) MMBTU MWh
Derivative [Line Items] |  |
Contract in a liability position | $ | $ 0 | $ 0
PNM | Commodity derivatives | Fair value hedging | Buy |  |
Derivative [Line Items] |  |
Economic Hedges (in mmbtu and mwh) | MMBTU | 100,000 | 100,000
PNM | Commodity derivatives | Fair value hedging | Sell |  |
Derivative [Line Items] |  |
Economic Hedges (in mmbtu and mwh) | MWh | 4,800 | 0

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Definition Nominal energy measure used to calculate payments on a derivative instrument. + References No definition available. + Details
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