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Regulatory and Rate Matters - TNMP Narrative (Details) |  | 1 Months Ended | 3 Months Ended | 6 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | --- | ---
May 30, 2018 USD ($) | Jul. 30, 2011 USD ($) | Jun. 30, 2018 USD ($) advanced_meter customer | Jun. 30, 2018 USD ($) advanced_meter customer | Dec. 31, 2017 USD ($) | Jul. 25, 2018 customer
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |
Unrecovered investment revenue | $ 21,800,000 |  |  |  |  |
TNMP |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |
Net regulatory liability |  |  |  |  | $ 14,400,000 |
Reduction to revenue |  |  | $ 1,200,000 | $ 2,700,000 |  |
Energy efficiency cost recovery, requested change amount | 5,700,000 |  |  |  |  |
Energy efficiency cost recovery, requested bonus | 900,000 |  | $ 800,000 | $ 800,000 |  |
TNMP | 2018 TNMP Rate Case |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |
Requested rate increase (decrease) | $ 25,900,000 |  |  |  |  |
Requested return on equity | 10.50% |  |  |  |  |
New rate rider recovery, amount | $ 7,700,000 |  |  |  |  |
Regulatory liabilities |  |  |  |  | $ 146,500,000 |
TNMP | 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 | 242,000 |  |
Recovery in cost through initial fees |  |  | $ 200,000 | $ 200,000 |  |
Ongoing annual expenses |  |  | 500,000 | 500,000 |  |
Approved non-standard metering ongoing expenses monthly charge |  |  | $ 36.78 | $ 36.78 |  |
Presumed number of customers that will elect non-standard meter service (in customers) | customer |  |  | 1,081 | 1,081 |  |
Minimum | TNMP | 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 | $ 63.97 |  |
Maximum | TNMP | 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 | $ 168.61 |  |
Subsequent event | TNMP | 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 |  |  |  |  |  | 97

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