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Fair Value of Derivative and Other Financial Instruments - Sale of Power (Details)	Dec. 31, 2017 $ / MWh	Jun. 30, 2016 $ / MWh MW
Palo Verde Nuclear Generating Station Unit 3 [Member] | PNMR and PNM [Member]
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 [Member] | PNMR and PNM [Member] | Commodity Contract [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative, average forward price (in dollars per MWh)		26
Palo Verde Nuclear Generating Station Unit 3 [Member] | PNMR and PNM [Member] | Commodity Contract [Member] | Subsequent Event [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative, average forward price (in dollars per MWh)	29
Palo Verde Nuclear Generating Station [Member] | Public Service Company of New Mexico [Member] | Firm Contract [Member]
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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- Definition

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- Definition

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