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Income Taxes - Additional Information (Detail) - USD ($) $ in Millions | 1 Months Ended | 3 Months Ended | 6 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | --- | ---
Mar. 31, 2020 | Dec. 31, 2017 | Mar. 31, 2018 | Jun. 30, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Jan. 01, 2020 | Jun. 30, 2019
Income Taxes [Line Items] |  |  |  |  |  |  |  |  |
Regulatory Liability |  |  |  | $ 57.4 | $ 54.0 |  |  |  | $ 65.4
Regulatory liability, expected flow back to customers |  |  | $ 47.1 | 47.1 |  | $ 48.9 |  |  |
Regulatory liability, expected pass back to ratepayers |  |  |  | 0.2 | 0.3 |  |  |  |
Net Operating Loss Carryforwards Utilized For Income Taxes |  |  |  |  |  | 5.7 |  |  |
Deferred tax assets, operating loss carryforwards, federal |  |  |  |  | 1.6 |  |  |  |
Alternative minimum tax credit carryforwards |  |  |  | 0.9 | 0.9 |  |  | $ 0.1 |
Additional Alternative minimum tax credit carryforwards |  |  |  |  | 1.9 |  |  |  |
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 |  |  |  |  |  | $ 2.0 |  |  |
Income Tax Related Liabilities |  |  |  |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |  |  |  |
Regulatory Liability |  | $ 48.9 |  | 45.9 | 47.6 |  | $ 48.9 |  | $ 47.8
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 |  |  |  |  |
Gas Ratepayers | Northern Utilities Inc |  |  |  |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |  |  |  |
Regulatory liability, expected flow back to customers | $ 12.3 |  |  |  |  |  |  |  |
Average Rate Assumption Method estimated flow back period | 20 years |  |  |  |  |  |  |  |
Tax Year 2018 |  |  |  |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |  |  |  |
Corporate federal income tax |  | 21.00% |  | 21.00% |  |  |  |  |
Tax Year 2019 |  |  |  |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |  |  |  |
Net Operating Loss Carryforwards Utilized For Income Taxes |  |  |  |  | $ 3.5 |  |  |  |

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