DERIVATIVE INSTRUMENTS - Unobservable Inputs Utilized (Details) - Fair Value, Inputs, Level 3 - $ / KWmo	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Minimum
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Percentage of Exit Price Premiums	4.20%
The Connecticut Light And Power Company | Minimum
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Capacity Prices (in USDPerKiloWattMonth)	4.30	5.00
Forward Reserve (in USDPerKiloWattMonth)	0.75	1.00
Percentage of Exit Price Premiums	15.70%
The Connecticut Light And Power Company | Maximum
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Capacity Prices (in USDPerKiloWattMonth)	7.44	8.70
Forward Reserve (in USDPerKiloWattMonth)	1.78	2.00