REGULATORY ACCOUNTING - Narrative (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	$ 255.6	$ 196.9
Regulatory costs	6,433.3	6,569.9
Storm Restoration Costs, Net
Regulatory Assets [Line Items]
Regulatory costs	745.9	765.6
COVID-19
Regulatory Assets [Line Items]
Net incremental deferred costs related to non-tracked uncollectible expense	39.1	15.8
Net incremental deferred costs	45.8	24.0
Tropical Storm Isaias | Storm Restoration Costs, Net
Regulatory Assets [Line Items]
Regulatory costs	243.0
CL&P
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	115.5	84.1
Regulatory costs	2,204.4	2,211.8
CL&P | Storm Restoration Costs, Net
Regulatory Assets [Line Items]
Regulatory costs	500.2	515.1
CL&P | COVID-19
Regulatory Assets [Line Items]
Net incremental deferred costs	13.2	4.7
CL&P | Tropical Storm Isaias | Storm Restoration Costs, Net
Regulatory Assets [Line Items]
Regulatory costs	225.0
NSTAR Electric
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	77.1	69.8
Regulatory costs	1,684.4	1,703.9
NSTAR Electric | Storm Restoration Costs, Net
Regulatory Assets [Line Items]
Regulatory costs	169.5	186.4
NSTAR Electric | COVID-19
Regulatory Assets [Line Items]
Net incremental deferred costs	15.8	11.9
PSNH
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	3.1	4.3
Regulatory costs	950.4	989.1
PSNH | Storm Restoration Costs, Net
Regulatory Assets [Line Items]
Regulatory costs	$ 76.2	$ 64.1