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Electric Operating Revenues - Narrative (Details) | Sep. 30, 2018 USD ($) utility MW | Dec. 31, 2017 USD ($) MW
--- | --- | ---
Disaggregation of Revenue [Line Items] |  |
Number of regulated utilities | utility | 2 |
Contract assets | $ | $ 0 |
PNM |  |
Disaggregation of Revenue [Line Items] |  |
Expected exposure to market risk (in megawatts) | 65 |
Power to be sold to third party (in megawatts) | 36 |
Customer contracts | PNM |  |
Disaggregation of Revenue [Line Items] |  |
Accounts receivable | $ | $ 76,900,000 | $ 61,800,000
Clean Air Act, SNCR | Palo Verde Nuclear Generating Station Unit 3 | PNM |  |
Disaggregation of Revenue [Line Items] |  |
Number of megawatts nuclear generation (in megawatts) |  | 134

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