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Regulatory and Rate Matters - TNMP Schedules (Details) - Texas-New Mexico Power Company [Member] - USD ($) $ in Millions |  |  |  | 6 Months Ended | 12 Months Ended
--- | --- | --- | --- | --- | ---
Sep. 08, 2016 | Mar. 23, 2016 | Sep. 10, 2015 | Sep. 09, 2015 | Mar. 15, 2015 | Sep. 07, 2014 | Feb. 28, 2018 | Feb. 28, 2017 | Feb. 29, 2016 | Feb. 28, 2015
Energy Efficiency [Member] |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |
Aggregate Collection Amount |  |  |  |  |  |  |  |  | $ 5.7 | $ 5.6
Performance Bonus |  |  |  |  |  |  |  |  | $ 1.5 | $ 0.7
Energy Efficiency [Member] | Scenario, Forecast [Member] |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |
Aggregate Collection Amount |  |  |  |  |  |  | $ 6.0 | $ 6.0 |  |
Performance Bonus |  |  |  |  |  |  | $ 0.8 | $ 0.7 |  |
Transmission Cost of Service Rates [Member] |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |
Approved Increase in Rate Base | $ 9.5 | $ 25.8 | $ 7.0 | $ 27.1 | $ 25.2 | $ 18.2 |  |  |  |
Annual Increase in Revenue | $ 1.8 | $ 4.3 | $ 1.4 | $ 4.4 | $ 4.2 | $ 2.9 |  |  |  |

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