DERIVATIVE INSTRUMENTS - Summary of Level 3 Derivative Contracts and Significant Unobservable Inputs Used (Details) - CL&P - Level 3 - $ / KWmo	Sep. 30, 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	5.12	7.34
Capacity Prices | Weighted Average
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative measurement input	4.57
Forward Reserve Prices | Minimum
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative measurement input	0.54	0.80
Forward Reserve Prices | Maximum
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative measurement input	0.90	1.90
Forward Reserve Prices | Weighted Average
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative measurement input	0.72