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Regulatory and Rate Matters - Transmission Cost of Service Rates (Details) - Transmission Cost of Service Rates [Member] - Texas-New Mexico Power Company [Member] - USD ($) $ in Millions | 6 Months Ended
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Mar. 09, 2016 | Sep. 15, 2015 | Mar. 15, 2015 | Sep. 07, 2014 | Mar. 12, 2014
Public Utilities, General Disclosures [Line Items] |  |  |  |  |
Approved Increase in Rate Base |  | $ 27.1 | $ 25.2 | $ 18.2 | $ 18.1
Annual Increase in Revenue |  | $ 4.4 | $ 4.2 | $ 2.9 | $ 2.8
Subsequent Event [Member] |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |
Approved Increase in Rate Base | $ 7.0 |  |  |  |
Annual Increase in Revenue | $ 1.4 |  |  |  |

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