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Fair Value of Derivative and Other Financial Instruments - Volume Positions and Requirements to Provide Collateral (Details) - Commodity Contract - Economic Hedges - PNMR and PNM | 12 Months Ended
--- | ---
Dec. 31, 2017 MMBTU MWh | Dec. 31, 2016 MMBTU MWh
Long |  |
Derivative [Line Items] |  |
Power-related contracts | MMBTU | 100,000 | 254,100
Short |  |
Derivative [Line Items] |  |
Power-related contracts | MWh | 0 | 2,471,600

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