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Fair Value of Derivative and Other Financial Instruments - Sale of Power (Details)	Dec. 31, 2017 $ / MWh	Mar. 31, 2017 MW
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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