Amount of contractual obligation, including, but not limited to, long-term debt, lease obligation, purchase obligation, and other commitments.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 942
-SubTopic 235
-Name Accounting Standards Codification
-Section S99
-Paragraph 2
-Subparagraph (SX 210.9-06)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477314/942-235-S99-2

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S50
-Paragraph 4
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480738/235-10-S50-4

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 3
-Subparagraph (SX 210.12-04(a))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480678/235-10-S99-3

Regulatory Matters - Additional Information (Detail 1) $ in Millions												1 Months Ended	3 Months Ended		6 Months Ended
	Jun. 04, 2026 USD ($)	Apr. 30, 2026 USD ($)	Apr. 15, 2026 USD ($)	Mar. 19, 2026	Apr. 15, 2025 USD ($)	Dec. 09, 2024 USD ($)	Oct. 04, 2024 USD ($)	Jul. 18, 2024 USD ($)	Jun. 28, 2024 USD ($)	Aug. 17, 2023 USD ($)	Jun. 25, 2019	Dec. 31, 2024 Megawatts	Jun. 30, 2026 USD ($)	Jun. 30, 2025 USD ($)	Jun. 30, 2026 USD ($) MW	Jun. 30, 2025 USD ($)	Jun. 30, 2027 MW MWh	Oct. 31, 2025 USD ($)	Nov. 25, 2024 USD ($)	Sep. 06, 2024 MW	Aug. 30, 2023 MW	May 31, 2019 MW	Jul. 31, 2018 MWh MW
Regulatory Assets [Line Items]
Remuneration Percentage											2.75%
Tax Benefit Cost Recovery	$ 0.9
Income Tax Expense (Benefit)	$ 2.8												$ 1.1	$ 0.7	$ 13.0	$ 10.2
Percentage of approved return on equity, reflecting on equity															9.45%
Fitchburg Gas and Electric Light Company
Regulatory Assets [Line Items]
Facility power capacity to be procured in the future \| MW																					400
Power generation capacity \| MWh																	9,450,000
Percentage of approved return on equity									9.40%
Percentage of approved return on equity, reflecting on equity									52.00%
Percentage of approved return on equity, reflecting on debt									48.00%
Accumulated deferred income taxes						$ 1.4		$ 1.4
Energy storage systems												5,000			1,268
Mid Duration Storage \| Megawatts												1,500
Fitchburg Gas and Electric Light Company \| Offshore Energy [Member]
Regulatory Assets [Line Items]
Facility power capacity to be procured in the future \| MW																				2,678	5,600
Power generation facility \| MW																	1,600						400
Fitchburg Gas and Electric Light Company \| Qualified Clean Energy [Member]
Regulatory Assets [Line Items]
Power generation capacity \| MWh																							9,554,940
Fitchburg Gas and Electric Light Company \| Maximum [Member]
Regulatory Assets [Line Items]
Extended long-term renewable contracts												30 years
Fitchburg Gas Company
Regulatory Assets [Line Items]
Percentage of approved return on equity										9.40%
Percentage of approved return on equity, reflecting on equity										52.00%
Percentage of approved return on equity, reflecting on debt										48.00%
Granite State
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue							$ 3.0								$ 1.3
Spending cap							$ 29.9
Revenue increase approved by regulator																			$ 1.2
Fitchburg - Grid Modernization
Regulatory Assets [Line Items]
Regulatory assets approved increase in revenue due to be recovered			$ 1.9		$ 1.4
Order providing cost			$ 2.0		$ 1.5
Fitchburg Gas System Enhancement Program
Regulatory Assets [Line Items]
Regulatory assets approved increase in revenue due to be recovered																		$ 5.4
Regulatory assets deferred cost																		1.0
Regulatory recovery amount																		$ 6.4
Fitchburg Gas System Enhancement Program \| Minimum [Member]
Regulatory Assets [Line Items]
Regulatory assets deferred cost		$ 0.7
Revenue cap		2.00%
Fitchburg Gas System Enhancement Program \| Maximum [Member]
Regulatory Assets [Line Items]
Regulatory assets deferred cost		$ 1.0
Revenue cap		2.50%
July First Two Thousand And Twenty Four [Member] \| Fitchburg Gas and Electric Light Company
Regulatory Assets [Line Items]
Approved annual increase in rates									$ 4.7	$ 6.8
July First Two Thousand And Twenty Four [Member] \| Fitchburg Gas Company
Regulatory Assets [Line Items]
Public utilities interim increase decrease amount										$ 10.9
Approved annual increase in rates									$ 10.1
Second Solicitation [Member] \| Fitchburg Gas and Electric Light Company \| Offshore Energy [Member]
Regulatory Assets [Line Items]
Power generation facility \| MW																						2,400
FERC Transmission Formula Rate Proceedings [Member]
Regulatory Assets [Line Items]
Percentage of approved return on equity		11.39%													9.57%
FERC Transmission Formula Rate Proceedings [Member] \| Minimum [Member]
Regulatory Assets [Line Items]
Percentage of approved return on equity				9.57%
FERC Transmission Formula Rate Proceedings [Member] \| Maximum [Member]
Regulatory Assets [Line Items]
Percentage of approved return on equity				10.57%
FERC Transmission Formula Rate Proceedings [Member] \| Fitchburg Gas Company
Regulatory Assets [Line Items]
Charges															$ 0.9
Gas Division [Member] \| Fitchburg Gas and Electric Light Company
Regulatory Assets [Line Items]
Accumulated deferred income taxes						0.6
Electric Division [Member] \| Fitchburg Gas and Electric Light Company
Regulatory Assets [Line Items]
Accumulated deferred income taxes						$ 0.8