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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

X

- Definition

Amount of allowance for credit loss on accounts receivable.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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-SubTopic 210
-Name Accounting Standards Codification
-Section S99
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
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X

- Definition

Noncurrent portion of the carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

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Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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