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Regulatory and Rate Matters - TNMP Narrative (Details) - USD ($) $ in Millions |  |  | 3 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | --- | ---
Nov. 02, 2018 | May 30, 2018 | Mar. 31, 2018 | Dec. 31, 2018 | Dec. 31, 2017 | Mar. 31, 2019 | Aug. 06, 2018
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |
Unrecovered investment revenue |  | $ 20.2 |  |  |  |  |
TNMP |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |
Net regulatory liability |  |  |  |  | $ 37.8 |  |
Reduction to revenue |  |  | $ 1.5 | $ 5.4 |  |  |
Energy efficiency cost recovery, requested change amount |  | 5.7 |  |  |  |  | $ 5.6
Energy efficiency cost recovery, requested bonus |  | 0.9 |  |  |  | $ 0.8 | $ 0.8
TNMP | 2018 TNMP Rate Case |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |
Requested rate increase (decrease) | $ 10.0 | $ 25.9 |  |  |  |  |
Requested return on equity | 9.65% | 10.50% |  |  |  |  |
Requested cost of debt | 6.44% | 7.20% |  |  |  |  |
Requested debt capital structure | 55.00% | 50.00% |  |  |  |  |
Requested equity capital structure | 45.00% | 50.00% |  |  |  |  |
Investments excluded from rate request | $ 10.6 |  |  |  |  |  |
Refund of federal income tax rates period |  |  |  |  | 5 years |  |

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