The increase (decrease) during the reporting period in the account that represents the temporary difference that results from Income or Loss that is recognized for accounting purposes but not for tax purposes and vice versa.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 45
-Paragraph 28
-Subparagraph (a)
-SubTopic 10
-Topic 230
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482740/230-10-45-28

Regulatory Matters - Additional Information (Detail) - USD ($) $ in Millions														3 Months Ended	6 Months Ended
	Jun. 01, 2026	May 01, 2026	Apr. 30, 2026	Apr. 01, 2026	Mar. 19, 2026	Feb. 27, 2026	Jan. 09, 2026	May 01, 2025	Dec. 09, 2024	Oct. 04, 2024	Jul. 18, 2024	Jun. 28, 2024	Aug. 17, 2023	Jun. 30, 2026	Jun. 30, 2026	Apr. 15, 2026	Oct. 31, 2025	Apr. 15, 2025
Regulatory Assets [Line Items]
Percentage of approved return on equity, reflecting on equity															9.45%
Net increase to pretax income														$ 1.7
Overall cost of capital percentage															9.32%
Increase Decrease In Deferred Revenue						$ 3.2
Settlement Agreement [Member]
Regulatory Assets [Line Items]
percent above present rates		5.40%
Transfer revenue amount		$ 5.5
Massachusetts Solar Massachusetts Renewable Target Program [Member]
Regulatory Assets [Line Items]
Program duration							20 years								20 years
FERC Transmission Formula Rate Proceedings [Member]
Regulatory Assets [Line Items]
Percentage of approved return on equity			11.39%												9.57%
Minimum [Member] \| Massachusetts Solar Massachusetts Renewable Target Program [Member]
Regulatory Assets [Line Items]
Program costs							$ 4,500.0
Minimum [Member] \| FERC Transmission Formula Rate Proceedings [Member]
Regulatory Assets [Line Items]
Percentage of approved return on equity					9.57%
Maximum \| Massachusetts Solar Massachusetts Renewable Target Program [Member]
Regulatory Assets [Line Items]
Estimate program cost							191.0
Program costs							$ 6,700.0
Maximum \| FERC Transmission Formula Rate Proceedings [Member]
Regulatory Assets [Line Items]
Percentage of approved return on equity					10.57%
Northern Utilities Inc
Regulatory Assets [Line Items]
Percentage of total operating revenues including cost of gas supply	7.40%
Approved annual increase in rates	$ 10.4
Northern Utilities-Base Rates-New Hampshire
Regulatory Assets [Line Items]
percent above present rates				5.90%
Transfer revenue amount				$ 6.1
Northern Utilities-Base Rates-New Hampshire \| Settlement Agreement [Member]
Regulatory Assets [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Percentage				9.60%
Requested annual increase in rates				$ 9.8
Unitil Energy Systems Inc
Regulatory Assets [Line Items]
Percentage of prior year total revenue															2.50%
Approved annual increase in rates								$ 13.0
Fitchburg Gas and Electric Light 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%
Accumulated deferred income taxes									$ 1.4		$ 1.4
Fitchburg Gas and Electric Light Company \| July 1, 2024
Regulatory Assets [Line Items]
Approved annual increase in rates												$ 4.7	$ 6.8
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%
Fitchburg Gas Company \| July 1, 2024
Regulatory Assets [Line Items]
Approved annual increase in rates												$ 10.1
Granite State
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue										$ 3.0					$ 1.3
Fitchburg - Grid Modernization
Regulatory Assets [Line Items]
Regulatory assets approved increase in revenue due to be recovered																$ 1.9		$ 1.4
Fitchburg Performance Base Rate Adjustment Electric
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue						1.1
Fitchburg Performance Base Rate Adjustment Gas
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue						$ 1.7
Fitchburg Gas System Enhancement Program
Regulatory Assets [Line Items]
Regulatory assets approved increase in revenue due to be recovered																	$ 5.4
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