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Summary of Significant Accounting Policies - Additional Information (Detail) $ in Millions | 6 Months Ended |  |
--- | --- | --- | ---
Jun. 30, 2024 USD ($) Subsidiary mi | Dec. 31, 2023 USD ($) | Jun. 30, 2023 USD ($)
Significant Accounting Policies [Line Items] |  |  |
Number of Subsidiaries | Entity | Subsidiary | 3 |  |
Length of Pipeline | mi | 85 |  |
Cost of removal obligation | $ 134.9 | $ 126.3 | $ 123.2
Investments in trading securities | 5.8 | 6.0 | 5.8
Allowance for doubtful accounts | 2.2 | 2.4 | 2.2
Regulatory assets | 114.1 | 109.6 | 108.3
Electric and Gas Division [Member] |  |  |
Significant Accounting Policies [Line Items] |  |  |
Price of Acquisition | 70.9 |  |
Other Deferred Charges [Member] |  |  |
Significant Accounting Policies [Line Items] |  |  |
Regulatory assets | 13.1 | 11.0 | 11.0
Unbilled Revenues [Member] |  |  |
Significant Accounting Policies [Line Items] |  |  |
Allowance for doubtful accounts | 0.1 | 0.1 | 0.1
Accounts Receivable [Member] |  |  |
Significant Accounting Policies [Line Items] |  |  |
Allowance for doubtful accounts | $ 2.2 | 2.3 | 2.2
Utilities |  |  |
Significant Accounting Policies [Line Items] |  |  |
Number of Subsidiaries | Entity | Subsidiary | 3 |  |
Fitchburg Gas And Electric Light Company [Member] | Other Deferred Charges [Member] | Electric and Gas Division [Member] |  |  |
Significant Accounting Policies [Line Items] |  |  |
Hardship accounts in regulatory assets | $ 7.2 | 6.0 | 6.0
Environmental and Rate Case Costs and Other Expenditures | Recovered over the next seven years |  |  |
Significant Accounting Policies [Line Items] |  |  |
Regulatory assets | 7.2 |  |
ISO-NE Obligations |  |  |
Significant Accounting Policies [Line Items] |  |  |
Cash Deposits | $ 2.3 | 3.3 | 3.3
Maximum |  |  |
Significant Accounting Policies [Line Items] |  |  |
Lease term | 12 months |  |
Deferred Compensation Plan [Member] |  |  |
Significant Accounting Policies [Line Items] |  |  |
Investments in trading securities | $ 1.9 | $ 1.3 | $ 1.1

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