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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