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	Jul. 01, 2025	May 01, 2025	Feb. 28, 2025	Dec. 09, 2024	Nov. 26, 2024	Nov. 01, 2024	Oct. 04, 2024	Sep. 11, 2024	Jul. 18, 2024	Jul. 16, 2024	Jun. 28, 2024	May 01, 2024	Sep. 20, 2023	Aug. 17, 2023	Jul. 26, 2023	Jul. 20, 2022	Jun. 08, 2022	Apr. 15, 2025	Oct. 31, 2024	May 31, 2024	May 31, 2023
Regulatory Assets [Line Items]
Requested annual increase in rates												$ 2.1
Northern Utilities Inc
Regulatory Assets [Line Items]
Approved annual increase in rates													$ 7.6
Return on equity													9.35%
Equity ratio													52.01%
Weighted average cost of capital													7.22%
Northern Utilities Inc \| Settlement Agreement [Member]
Regulatory Assets [Line Items]
Approved annual increase in rates																$ 6.1
Northern Utilities Inc \| New Hampshire
Regulatory Assets [Line Items]
Percentage of approved return on equity																9.30%
Percentage of approved return on equity, reflecting on equity																52.00%
Percentage of approved return on equity, reflecting on debt																48.00%
Requested annual increase in rates																	$ 1.6
Unitil Energy Systems Inc
Regulatory Assets [Line Items]
percent above present rates	3.70%	7.30%
Approved annual increase in rates		$ 18.5
Public utilities approved increase amount of annual revenue to recover eligible capital investments	$ 7.8
Fitchburg Gas and Electric Light Company
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue						$ 0.5		$ 3.5							$ 3.1
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%
Increase in cost										$ 0.8	$ 2.2
Accumulated deferred income taxes				$ 1.4	$ 0.1				$ 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]
Increase (decrease) in annual revenue														4.2
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%
Approved annual increase in rates											$ 10.1
Increase in cost										$ 0.9	$ 4.9
Accumulated deferred income taxes				1.4					$ 1.4
Fitchburg Gas Company \| Maximum
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue														$ 6.7
Granite State
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue							$ 3.0
Fitchburg - Grid Modernization
Regulatory Assets [Line Items]
Regulatory assets approved increase in revenue due to be recovered																		$ 1.4		$ 1.3	$ 1.0
Fitchburg Performance Base Rate Adjustment Electric
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue			$ 1.6
Fitchburg Performance Base Rate Adjustment Gas
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue			$ 0.7
Fitchburg Gas System Enhancement Program
Regulatory Assets [Line Items]
Regulatory assets approved increase in revenue due to be recovered																			$ 3.5
Gas Division [Member]
Regulatory Assets [Line Items]
Accumulated deferred income taxes				(0.6)
Gas Division [Member] \| Fitchburg Gas and Electric Light Company
Regulatory Assets [Line Items]
Accumulated deferred income taxes				(0.6)
Electric Division [Member]
Regulatory Assets [Line Items]
Accumulated deferred income taxes				(0.8)
Electric Division [Member] \| Fitchburg Gas and Electric Light Company
Regulatory Assets [Line Items]
Accumulated deferred income taxes				$ (0.8)