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Regulatory and Rate Matters - Firm-Requirements Wholesale Customers (Details) - Public Service Company of New Mexico - Firm Requirements Wholesale Power Rate Case, Navopache $ in Millions | 12 Months Ended
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Dec. 31, 2017 USD ($) MW | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($)
Public Utilities, General Disclosures [Line Items] |  |  |
Serving megawatts of load under a lower tariff (in mw) | MW | 10 |  |
Revenue for power sold under specific contract | $ | $ 4.5 | $ 20.0 | $ 27.1

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- Details
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