REGULATORY ACCOUNTING - Narrative (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	$ 215.5	$ 196.9
Regulatory costs	6,512.6	6,569.9
CL&P
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	98.0	84.1
Regulatory costs	2,250.0	2,211.8
NSTAR Electric
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	66.5	69.8
Regulatory costs	1,704.4	1,703.9
PSNH
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	3.7	4.3
Regulatory costs	977.2	989.1
COVID-19
Regulatory Assets [Line Items]
Net incremental deferred costs	33.8	24.0
Net incremental deferred costs related to non-tracked uncollectible expense	27.3	15.8
COVID-19 | CL&P
Regulatory Assets [Line Items]
Net incremental deferred costs	8.8	4.7
COVID-19 | NSTAR Electric
Regulatory Assets [Line Items]
Net incremental deferred costs	13.6	11.9
COVID-19 | PSNH
Regulatory Assets [Line Items]
Net incremental deferred costs	0.6	1.1
Storm Restoration Costs, Net
Regulatory Assets [Line Items]
Regulatory costs	789.5	765.6
Storm Restoration Costs, Net | CL&P
Regulatory Assets [Line Items]
Regulatory costs	529.8	515.1
Storm Restoration Costs, Net | NSTAR Electric
Regulatory Assets [Line Items]
Regulatory costs	184.3	186.4
Storm Restoration Costs, Net | PSNH
Regulatory Assets [Line Items]
Regulatory costs	75.4	$ 64.1
Storm Restoration Costs, Net | Tropical Storm Isaias
Regulatory Assets [Line Items]
Regulatory costs	249.0
Storm Restoration Costs, Net | Tropical Storm Isaias | CL&P
Regulatory Assets [Line Items]
Regulatory costs	$ 231.0