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Regulatory and Rate Matters - Transmission Cost of Service Rates (Details) - Texas-New Mexico Power Company [Member] - Transmission Cost of Service Rates [Member] - USD ($) $ in Millions | 6 Months Ended
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Mar. 15, 2017 | Sep. 09, 2016 | Mar. 22, 2016 | Sep. 09, 2015 | Mar. 15, 2015
Public Utilities, General Disclosures [Line Items] |  |  |  |  |
Approved Increase in Rate Base |  | $ 25.8 | $ 7.0 | $ 27.1 | $ 25.2
Annual Increase in Revenue |  | $ 4.3 | $ 1.4 | $ 4.4 | $ 4.2
Subsequent event [Member] |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |
Approved Increase in Rate Base | $ 9.5 |  |  |  |
Annual Increase in Revenue | $ 1.8 |  |  |  |

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