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Regulatory and Rate Matters - TNMP Schedules (Details) - Texas-New Mexico Power Company - USD ($) $ in Millions |  |  |  |  |  | 6 Months Ended | 12 Months Ended
--- | --- | --- | --- | --- | --- | --- | ---
Sep. 13, 2017 | Mar. 14, 2017 | Sep. 08, 2016 | Mar. 23, 2016 | Sep. 10, 2015 | Sep. 09, 2015 | Feb. 28, 2019 | Feb. 28, 2018 | Feb. 28, 2017 | Feb. 29, 2016
Energy efficiency costs |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |
Aggregate Collection Amount |  |  |  |  |  |  |  |  | $ 6.0 | $ 5.7
Performance Bonus |  |  |  |  |  |  |  |  | $ 0.7 | $ 1.5
Energy efficiency costs | Scenario, forecast |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |
Aggregate Collection Amount |  |  |  |  |  |  | $ 6.0 | $ 6.0 |  |
Performance Bonus |  |  |  |  |  |  | $ 1.1 | $ 0.8 |  |
Transmission Cost of Service Rates |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |
Approved Increase in Rate Base | $ 27.5 | $ 30.2 | $ 9.5 | $ 25.8 | $ 7.0 | $ 27.1 |  |  |  |
Annual Increase in Revenue | $ 4.7 | $ 4.8 | $ 1.8 | $ 4.3 | $ 1.4 | $ 4.4 |  |  |  |

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