DERIVATIVE INSTRUMENTS - Unobservable Inputs Utilized (Details) - The Connecticut Light And Power Company - Fair Value, Inputs, Level 3 - $ / KWmo	Dec. 31, 2017	Dec. 31, 2016
Minimum
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Capacity Prices (in USDPerKiloWattMonth)	5.00	5.50
Forward Reserve (in USDPerKiloWattMonth)	1.00	1.40
Maximum
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Capacity Prices (in USDPerKiloWattMonth)	8.70	8.70
Forward Reserve (in USDPerKiloWattMonth)	2.00	2.00