REGULATORY ACCOUNTING - Narrative (Details) - USD ($) $ in Millions				3 Months Ended	9 Months Ended
	Jul. 25, 2025	Aug. 01, 2024	Jun. 11, 2024	Sep. 30, 2025	Sep. 30, 2025	Dec. 31, 2027	Dec. 31, 2026	Sep. 19, 2025	Sep. 10, 2025	Aug. 01, 2025	Jul. 01, 2025	Dec. 31, 2024
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved				$ 244.2	$ 244.2							$ 221.0
Regulatory costs				112.8	112.8							92.5
CL&P
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved				121.8	121.8							116.3
Regulatory costs				50.2	50.2							47.2
General obligation bonds								$ 107.8			$ 125.0
Reduce costs of hardship protection measures charged to retail customers (percent)											67.00%
New general obligation bonds to fund the electric vehicle charging program (percentage)											80.00%
CL&P | Forecast
Regulatory Assets [Line Items]
New general obligation bonds to fund the electric vehicle charging program						$ 20.0	$ 30.0
PSNH
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved				5.3	$ 5.3							4.5
Implement a temporary annual base distribution rate increase		$ 61.2
Permanent base distribution rates, amount			$ 181.9
Final proposed rate increase			$ 103.0
Distribution revenue										$ 519.0
Permanent annual base distribution rate increase	$ 100.7
Public utilities, approved return on equity (percentage)			9.50%		9.50%
Public utilities, approved equity capital structure (percentage)			50.00%		75.00%
Pre-tax benefit to earnings, recoupment									$ 9.1
Unrecovered storm costs			$ 247.0
Impact of the rate, pre-tax benefit to earnings				15.6
PSNH | Minimum
Regulatory Assets [Line Items]
Storm costs recovered in base rates			12.0
PSNH | Maximum
Regulatory Assets [Line Items]
Storm costs recovered in base rates			$ 19.0
NSTAR Electric
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved				46.2	$ 46.2							41.1
Regulatory costs				$ 31.1	$ 31.1							$ 24.4