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Regulatory and Rate Matters - TNMP Narrative (Details) - TNMP $ in Millions |  |  |  |  |  | 1 Months Ended | 12 Months Ended |  |  |  |  |  |  |
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Jun. 26, 2020 USD ($) | Apr. 14, 2020 USD ($) | Apr. 06, 2020 USD ($) | Dec. 20, 2018 USD ($) | May 30, 2018 USD ($) | Oct. 31, 2019 USD ($) | Aug. 31, 2019 USD ($) | Dec. 31, 2017 USD ($) | Oct. 02, 2020 USD ($) | Sep. 30, 2020 USD ($) | Jul. 27, 2020 USD ($) | May 29, 2020 USD ($) | Mar. 30, 2020 MWh | Mar. 01, 2020 USD ($) | Dec. 31, 2019 USD ($)
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net regulatory liability |  |  |  |  |  |  |  | $ 37.8 |  |  |  |  |  |  |
Requested increase in annual distribution revenue |  |  | $ 14.7 |  |  |  |  |  |  |  |  |  |  |  |
Incremental distribution investments |  |  | $ 149.2 |  |  |  |  |  |  |  |  |  |  |  |
Annual distribution revenue requirement | $ 14.3 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Energy efficiency cost recovery, requested change amount |  |  |  |  |  |  |  |  |  |  |  | $ 5.9 |  | $ 5.9 |
Energy efficiency cost recovery, requested bonus |  |  |  |  |  |  |  |  |  |  | $ 1.0 | $ 1.0 |  | $ 0.8 |
Rider charge from electricity relief program (per MWh) | MWh |  |  |  |  |  |  |  |  |  |  |  |  | 0.33 |  |
Proceeds from short-term debt |  | $ 0.5 |  |  |  |  |  |  |  |  |  |  |  |  |
Subsequent event |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public utilities, estimate of possible upgrades |  |  |  |  |  |  |  |  | $ 46.0 |  |  |  |  |  |
COVID-19 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory liability |  |  |  |  |  |  |  |  |  | $ 0.5 |  |  |  |  |
Other COVID-19 related costs |  |  |  |  |  |  |  |  |  | $ 0.5 |  |  |  |  |
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 |  |  |  |  |  |  |
Recovery of costs incurred |  |  |  |  |  | $ 3.3 | $ 3.8 |  |  |  |  |  |  |  |
Recovery costs collection period |  |  |  |  |  |  | 3 years |  |  |  |  |  |  |  |
Write off of regulatory disallowance |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0.5

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