REGULATORY ACCOUNTING - Narrative (Details) - USD ($) $ in Millions	Mar. 31, 2026	Dec. 31, 2025
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	$ 247.4	$ 244.2
Regulatory costs	123.7	123.2
Regulatory liabilities	6,063.6	5,538.1
FERC ROE Complaints
Regulatory Assets [Line Items]
Regulatory liabilities	60.4
CL&P
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	130.3	127.1
Regulatory costs	60.9	57.0
Regulatory liabilities	2,104.3	1,862.6
CL&P | FERC ROE Complaints
Regulatory Assets [Line Items]
Regulatory liabilities	28.7
NSTAR Electric
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	47.4	51.0
Regulatory costs	30.7	34.0
Regulatory liabilities	2,557.8	2,352.9
NSTAR Electric | FERC ROE Complaints
Regulatory Assets [Line Items]
Regulatory liabilities	24.2
PSNH
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	5.1	5.4
Regulatory liabilities	559.2	$ 535.8
PSNH | FERC ROE Complaints
Regulatory Assets [Line Items]
Regulatory liabilities	$ 7.5