COMMITMENTS AND CONTINGENCIES - Long-Term Contractual Arrangement Narrative (Details)	12 Months Ended
	Dec. 31, 2017 contract project MW	Dec. 31, 2008 contract MW
Long-term Purchase Commitment [Line Items]
Number of capacity contracts for differences between CL&P and United Illuminated Company | contract	4
Amount of capacity contracts for differences between CL&P and United Illuminated Company (in MW) | MW	787
Number of generation projects between CL&P and United Illuminated Company | project	3
Number of demand response projects between CL&P and United Illuminated Company | project	1
Percentage of costs and benefits borne by United Illuminated Company	20.00%
Peaker Contracts For Differences
Long-term Purchase Commitment [Line Items]
Percentage of costs and benefits borne by United Illuminated Company		20.00%
Period of payment to generation facility owner		30 years
The Connecticut Light And Power Company | Peaker Contracts For Differences
Long-term Purchase Commitment [Line Items]
Number of capacity contracts | contract		3
Amount of capacity contracts | MW		500
Percentage of costs and benefits borne		80.00%
CL&P, NSTAR Electric and PSNH | Transmission Support Commitments
Long-term Purchase Commitment [Line Items]
Period of payment of annual operation and maintenance expense	30 years