DERIVATIVE INSTRUMENTS - Unobservable Inputs Utilized (Details) - The Connecticut Light and Power Company - Level 3 - $ / KWmo	Dec. 31, 2020	Dec. 31, 2019
Capacity Prices | Minimum
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative measurement input	4.30	3.01
Capacity Prices | Maximum
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative measurement input	5.30	7.34
Forward Reserve | Minimum
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative measurement input	0.54	0.80
Forward Reserve | Maximum
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative measurement input	0.90	1.90