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Regulatory and Rate Matters - Transmission Cost of Service Rates (Details) - Texas-New Mexico Power Company - Transmission Cost of Service Rates - USD ($) $ in Millions |  | 6 Months Ended
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Mar. 14, 2017 | Mar. 15, 2017 | Sep. 09, 2016 | Mar. 22, 2016
Public Utilities, General Disclosures [Line Items] |  |  |  |
Approved Increase in Rate Base | $ 30.2 | $ 9.5 | $ 25.8 | $ 7.0
Annual Increase in Revenue | $ 4.8 | $ 1.8 | $ 4.3 | $ 1.4

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