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Fair Value of Derivative and Other Financial Instruments - Sale of Power (Details) | Dec. 31, 2017 $ / MWh | Mar. 31, 2017 MW
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Palo Verde Nuclear Generating Station Unit 3 | PNMR and PNM |  |
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |
Number of megawatts nuclear generation (in megawatts) | MW |  | 134
Palo Verde Nuclear Generating Station Unit 3 | PNMR and PNM | Commodity derivatives | Subsequent event |  |
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |
Derivative, average forward price (in dollars per MWh) | $ / MWh | 29 |
Palo Verde Nuclear Generating Station | PNM | Contract |  |
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |
Percentage of electric power plant output sold |  | 100.00%

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