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Regulatory and Rate Matters - Firm-Requirements Wholesale Customers (Details) - Public Service Company of New Mexico [Member] $ in Millions |  | 3 Months Ended | 6 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Dec. 06, 2012 USD ($) | Sep. 30, 2015 USD ($) | Jun. 30, 2014 USD ($) | Dec. 31, 2014 USD ($) MW
Firm Requirements Wholesale Power Rate Case, Navopache [Member] [Member] |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |
Public utilities, approved rate increase (decrease) | $ 5.3 |  |  |
Public utilities, average monthly usage (in megawatts) | MW |  |  |  | 55
Public utilities, revenue for power sold under specific contract |  |  |  | $ 28.4
Firm Requirements Wholesale Power Rate Case [Member] |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |
Public utilities, contract extension | 10 years |  |  |
City of Gallup, New Mexico Contract [Member] |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |
Public utilities, revenue for power sold under specific contract |  |  | $ 6.1 |
Public utilities, gain on sale of substation |  | $ 1.1 |  |

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