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Income Taxes - Additional Information (Detail) - USD ($) $ in Millions | 1 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | ---
Dec. 31, 2017 | Dec. 31, 2019 | Sep. 30, 2019 | Dec. 31, 2020 | Dec. 31, 2018 | Dec. 31, 2017 | Sep. 30, 2018
Income Taxes [Line Items] |  |  |  |  |  |  |
Regulatory Liability |  |  | $ 61.1 |  | $ 58.5 |  | $ 60.1
Regulatory liability, expected flow back to customers |  |  | 47.1 |  |  | $ 48.9 |
Tax provision |  |  |  |  | $ 2.4 |  |
NOL carryforward assets expiration date |  |  |  |  | 2029 |  |
Alternative minimum tax credit carryforwards |  |  |  |  | $ 3.1 |  |
Scenario Forecast |  |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |  |
Regulatory liability, expected pass back to ratepayers |  | $ 0.4 |  | $ 1.6 |  |  |
Reconciling Mechanisms |  |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |  |
Regulatory liability, expected pass back to ratepayers |  |  |  |  |  | 1.8 |
Net Operating Loss Carryforward Assets |  |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |  |
Regulatory liability, expected pass back to ratepayers |  |  |  |  |  | 5.8 |
Tax Cuts And Jobs Act Of 2017 Change In Tax Rate Regulatory Liability recognized |  |  | $ 1.4 |  |  |  |
Minimum |  |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |  |
Average Rate Assumption Method estimated flow back period |  |  | 15 years |  |  |  |
Maximum |  |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |  |
Average Rate Assumption Method estimated flow back period |  |  | 20 years |  |  |  |
Fitchburg Gas and Electric Light Company |  |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |  |
Average Rate Assumption Method estimated flow back period |  |  | 15 years |  |  |  |
Income Tax Related Liabilities |  |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |  |
Regulatory Liability | $ 48.9 |  | $ 47.6 |  | $ 47.0 | $ 48.9 | $ 47.9
Electric Ratepayers | Fitchburg Gas and Electric Light Company |  |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |  |
Regulatory liability, expected flow back to customers |  |  | 10.1 |  |  |  |
Gas Ratepayers | Fitchburg Gas and Electric Light Company |  |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |  |
Regulatory liability, expected flow back to customers |  |  | $ 10.4 |  |  |  |
Tax Year 2018 |  |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |  |
Corporate federal income tax | 21.00% |  | 21.00% |  |  |  |
Internal Revenue Service [Member] | Tax Year 2018 |  |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |  |
Net Operating Loss Carryforwards Utilized For Income Taxes |  |  | $ 5.7 |  |  |  |
Federal, Maine, Massachusetts, and New Hampshire Tax Authorities |  |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |  |
Tax examination description |  |  | The Company remains subject to examination by Maine, Massachusetts, and New Hampshire tax authorities for the tax periods ended December 31, 2016; December 31, 2017; and December 31, 2018. |  |  |  |
Domestic Country And State Jurisdiction [Member] |  |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |  |
Net Operating Loss Carryforwards |  |  | $ 11.9 |  |  |  |

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