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Regulatory and Rate Matters (Tables) | 3 Months Ended
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Mar. 31, 2015
Regulated Operations [Abstract] |
Schedule of Rate Increases for Transmission Costs | The following sets forth TNMP’s most recent interim transmission cost rate increases:
Effective Date |  | Approved Increase in Rate Base |  | Annual Increase in Revenue
|  | (in millions)
September 17, 2013 |  | $ | 18.1 |  |  | $ | 2.8 |
March 13, 2014 |  | 18.2 |  |  | 2.9 |
September 8, 2014 |  | 25.2 |  |  | 4.2 |
March 16, 2015 |  | 27.1 |  |  | 4.4 |

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- Definition Schedule of Rate Increases for Transmission Costs [Table Text Block] + References No definition available. + Details
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