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Regulatory and Rate Matters - TNMP Narrative (Details) - TNMP | 1 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | ---
Jul. 30, 2011 USD ($) | Dec. 31, 2017 USD ($) advanced_meter | Apr. 23, 2018 customer | Mar. 31, 2018 USD ($) customer
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |
TCJA Regulatory liability for deferred income taxes |  | $ 146,500,000 |  |
Regulatory liabilities |  |  |  | $ 1,500,000
Advanced Meter System Deployment and Surcharge Request |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |
Approved deployment costs | $ 113,400,000 |  |  |
Collection of deployment costs through surcharge period | 12 years |  |  |
Number of advanced meters installed (more than) | advanced_meter |  | 242,000 |  |
Recovery in cost through initial fees |  |  |  | 200,000
Ongoing annual expenses |  |  |  | 500,000
Approved non-standard metering ongoing expenses monthly charge |  |  |  | $ 36.78
Presumed number of customers that will elect non-standard meter service (in customers) | customer |  |  |  | 1,081
Minimum | Advanced Meter System Deployment and Surcharge Request |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |
Approved non-standard metering service cost, initial fee range |  |  |  | $ 63.97
Maximum | Advanced Meter System Deployment and Surcharge Request |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |
Approved non-standard metering service cost, initial fee range |  |  |  | $ 168.61
Subsequent event | Advanced Meter System Deployment and Surcharge Request |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |
Current number of customers that have elected non-standard meter service (in customers) | customer |  |  | 98 |

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