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Regulatory and Rate Matters Regulatory and Rate Matters (Tables) | 12 Months Ended
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Dec. 31, 2016
Regulated Operations [Abstract] |
Schedule of Rate Increases for Transmission Costs | TNMP recovers the costs of its energy efficiency programs through an energy efficiency cost recovery factor, which includes projected program costs, under or over collected costs from prior years, rate case expenses, and performance bonuses (if the programs exceed mandated savings goals). The following sets forth TNMP’s energy efficiency cost recovery factor increases:
Effective Date |  | Aggregate Collection Amount |  | Performance Bonus
|  | (In millions)
March 1, 2014 |  | $ | 5.6 |  |  | $ | 0.7 |
March 1, 2015 |  | 5.7 |  |  | 1.5 |
March 1, 2016 |  | 6.0 |  |  | 0.7 |
March 1, 2017 |  | 6.0 |  |  | 0.8 |
TNMP can update its transmission rates twice per year to reflect changes in its invested capital. Updated rates reflect the addition and retirement of transmission facilities, including appropriate depreciation, federal income tax and other associated taxes, and the approved rate of return on such facilities. 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 13, 2014 |  | $ | 18.2 |  |  | $ | 2.9 |
September 8, 2014 |  | 25.2 |  |  | 4.2 |
March 16, 2015 |  | 27.1 |  |  | 4.4 |
September 10, 2015 |  | 7.0 |  |  | 1.4 |
March 23, 2016 |  | 25.8 |  |  | 4.3 |
September 8, 2016 |  | 9.5 |  |  | 1.8 |

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