REGULATORY ACCOUNTING - Narrative (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Regulatory Assets [Line Items]
Regulatory costs	$ 5,646.1	$ 5,514.7
Amount of regulatory costs not yet approved	183.9	146.0
CL&P
Regulatory Assets [Line Items]
Regulatory costs	2,006.5	1,735.9
Amount of regulatory costs not yet approved	84.2	51.8
PSNH
Regulatory Assets [Line Items]
Regulatory costs	912.4	945.8
Amount of regulatory costs not yet approved	15.3	18.0
NSTAR Electric
Regulatory Assets [Line Items]
Regulatory costs	1,472.7	1,535.6
Amount of regulatory costs not yet approved	55.0	55.7
COVID-19
Regulatory Assets [Line Items]
Deferred costs	15.1
Deferred costs related to uncollectible expense incurred at our natural gas distribution segment	11.4
Storm Restoration Costs
Regulatory Assets [Line Items]
Regulatory costs	739.6	540.6
Storm Restoration Costs | CL&P
Regulatory Assets [Line Items]
Regulatory costs	510.7	274.6
Storm Restoration Costs | PSNH
Regulatory Assets [Line Items]
Regulatory costs	57.5	65.4
Storm Restoration Costs | NSTAR Electric
Regulatory Assets [Line Items]
Regulatory costs	171.4	$ 200.6
Storm Restoration Costs | Tropical Storm Isaias
Regulatory Assets [Line Items]
Regulatory costs	276.0
Storm Restoration Costs | Tropical Storm Isaias | CL&P
Regulatory Assets [Line Items]
Regulatory costs	$ 257.0