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Regulatory and Rate Matters (Tables) | 9 Months Ended
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Sep. 30, 2017
Regulated Operations [Abstract] |
Schedule of Energy/Capacity Transactions | Information about the purchases and sales is as follows:
| Sales |  | Purchases
| GWh |  | Amount |  | GWh |  | Amount
|  |  | (In millions) |  |  |  | (In millions)
Three months ended September 30, 2017 | 202.4 |  |  | $ | 7.2 |  |  | 215.1 |  |  | $ | 7.6 |
Three months ended September 30, 2016 | 208.2 |  |  | 6.2 |  |  | 216.4 |  |  | 6.4 |
Nine months ended September 30, 2017 | 615.0 |  |  | 17.7 |  |  | 632.5 |  |  | 18.2 |
Nine months ended September 30, 2016 | 268.5 |  |  | 7.8 |  |  | 278.8 |  |  | 8.1 |
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)
September 10, 2015 |  | $ | 7.0 |  |  | $ | 1.4 |
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 |

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