DERIVATIVE INSTRUMENTS - Unobservable Inputs Utilized (Details) - The Connecticut Light And Power Company - Level 3 - $ / KWmo	Mar. 31, 2018	Dec. 31, 2017
Minimum
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Capacity Prices (in USDPerKiloWattMonth)	4.30	5.00
Forward Reserve (in USDPerKiloWattMonth)	0.95	1.00
Maximum
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Capacity Prices (in USDPerKiloWattMonth)	5.99	8.70
Forward Reserve (in USDPerKiloWattMonth)	1.00	2.00