Fair Value of Derivative and Other Financial Instruments - Sale of Power (Details) - Palo Verde Nuclear Generating Station Unit 3 - PNM	Sep. 30, 2017 $ / MWh MW
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Number of megawatts nuclear generation (in megawatts) | MW	134
Commodity derivatives
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