Summary of Significant Accounting Policies - Additional Information (Detail) $ in Millions	9 Months Ended
Sep. 30, 2024 USD ($) Subsidiary mi	Dec. 31, 2023 USD ($)	Sep. 30, 2023 USD ($)
Significant Accounting Policies [Line Items]
Number of Subsidiaries \| Entity \| Subsidiary	3
Length of Pipeline \| mi	85
Cost of removal obligation	$ 139.3	$ 126.3	$ 125.8
Investments in trading securities	5.8	6.0	5.9
Allowance for doubtful accounts	2.0	2.4	2.0
Regulatory assets	110.8	109.6	104.4
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	12.4	11.0	10.6
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	$ 1.9	2.3	2.0
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	$ 6.8	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	$ 5.8	3.3	3.5
Maximum
Significant Accounting Policies [Line Items]
Lease term	12 months
Deferred Compensation Plan [Member]
Significant Accounting Policies [Line Items]
Investments in trading securities	$ 2.1	$ 1.3	$ 1.1

Amount of allowance for credit loss on accounts receivable.

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The amount for the individual regulatory asset as itemized in a table of regulatory assets as of the end of the period.

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Length of natural gas transmission pipeline.

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