DERIVATIVE INSTRUMENTS - Unobservable Inputs Utilized (Details) - The Connecticut Light and Power Company - Level 3 - $ / KWmo	Sep. 30, 2018	Dec. 31, 2017
Minimum
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Capacity Prices (in USD per kilowatt month)	4.30	5.00
Forward Reserve (in USD per kilowatt month)	0.95	1.00
Maximum
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Capacity Prices (in USD per kilowatt month)	7.44	8.70
Forward Reserve (in USD per kilowatt month)	2.00	2.00