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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
	Dec. 31, 2017 USD ($) MW	Dec. 31, 2016 USD ($)
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

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Public Utilities, Expected average monthly usage in megawatts

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Public Utilities, Revenue For Power Sold Under Specific Contract

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