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COMMITMENTS AND CONTINGENCIES - Guarantees and Obligations Narrative (Details) - USD ($)
Sep. 30, 2021
Sep. 16, 2020
Loss Contingencies [Line Items]    
Fair value of guarantees $ 5,700,000  
Letter of Credit | Eversource Investment LLC    
Loss Contingencies [Line Items]    
Maximum borrowing capacity   $ 25,000,000