DERIVATIVE INSTRUMENTS - Unobservable Inputs Utilized (Details) - Fair Value, Inputs, Level 3 [Member] - $ / KWmo	Dec. 31, 2016	Dec. 31, 2015
Minimum [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Capacity Prices (in USDPerKiloWattMonth)	5.5	10.81
Maximum [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Capacity Prices (in USDPerKiloWattMonth)	8.7	15.82
The Connecticut Light And Power Company [Member] | Minimum [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Capacity Prices (in USDPerKiloWattMonth)	5.50	10.81
The Connecticut Light And Power Company [Member] | Maximum [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Capacity Prices (in USDPerKiloWattMonth)	8.70	12.60
Eversource and The Connecticut Light And Power Company [Member] | Minimum [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Forward Reserve (in USDPerKiloWattMonth)	1.40	2.00
Eversource and The Connecticut Light And Power Company [Member] | Maximum [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Forward Reserve (in USDPerKiloWattMonth)	2.00	2.00
Eversource and NSTAR Electric Company [Member] | Minimum [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
REC Prices (in USDPerKiloWattMonth)	24	45
Eversource and NSTAR Electric Company [Member] | Maximum [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
REC Prices (in USDPerKiloWattMonth)	29	51