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Regulatory and Rate Matters - TNMP Narrative (Details) $ in Millions |  |  | 1 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | --- | --- | ---
Nov. 02, 2018 USD ($) | May 30, 2018 USD ($) | Jul. 30, 2011 USD ($) | Sep. 30, 2018 USD ($) advanced_meter | Sep. 30, 2018 USD ($) advanced_meter | Dec. 31, 2017 USD ($) | Aug. 06, 2018 USD ($)
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |
Unrecovered investment revenue |  | $ 21.8 |  |  |  |  |
TNMP |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |
Net regulatory liability |  |  |  |  |  | $ 37.8 |
Reduction to revenue |  |  |  | $ 1.5 | $ 4.2 |  |
Energy efficiency cost recovery, requested change amount |  | 5.7 |  |  |  |  | $ 5.6
Energy efficiency cost recovery, requested bonus |  | 0.9 |  | $ 0.8 | $ 0.8 |  | $ 0.8
TNMP | 2018 TNMP Rate Case |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |
Requested rate increase (decrease) |  | $ 25.9 |  |  |  |  |
Requested return on equity |  | 10.50% |  |  |  |  |
Requested cost of debt |  | 7.20% |  |  |  |  |
Requested debt capital structure |  | 50.00% |  |  |  |  |
Requested equity capital structure |  | 50.00% |  |  |  |  |
New rate rider recovery, amount |  | $ 7.7 |  |  |  |  |
Regulatory liabilities |  |  |  |  |  | 146.5 |
Net regulatory liability |  |  |  |  |  | $ 14.4 |
TNMP | Advanced Meter System Deployment and Surcharge Request |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |
Approved deployment costs |  |  | $ 113.4 |  |  |  |
Collection of deployment costs through surcharge period |  |  | 12 years |  |  |  |
Number of advanced meters installed (more than) | advanced_meter |  |  |  | 242,000 | 242,000 |  |
Subsequent event | TNMP | 2018 TNMP Rate Case |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |
Requested rate increase (decrease) | $ 10.0 |  |  |  |  |  |
Requested return on equity | 9.65% |  |  |  |  |  |
Requested cost of debt | 6.44% |  |  |  |  |  |
Requested debt capital structure | 55.00% |  |  |  |  |  |
Requested equity capital structure | 45.00% |  |  |  |  |  |
Investments excluded from rate request | $ 11.7 |  |  |  |  |  |

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