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Regulatory and Rate Matters (Tables) | 3 Months Ended
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Mar. 31, 2018
Regulated Operations [Abstract] |
Schedule of Rate Increases for Transmission Costs | The following sets forth TNMP’s recent interim transmission cost rate increases:
Effective Date |  | Approved Increase in Rate Base |  | Annual Increase in Revenue
|  | (In millions)
March 23, 2016 |  | $ | 25.8 |  |  | $ | 4.3 |
September 8, 2016 |  | 9.5 |  |  | 1.8 |
March 14, 2017 |  | 30.2 |  |  | 4.8 |
September 13, 2017 |  | 27.5 |  |  | 4.7 |
March 27, 2018 |  | 32.0 |  |  | 0.6 |

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