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COMMITMENTS AND CONTINGENCIES - Schedule of Reconciliation of Activity in Environmental Reserve (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Accrual for Environmental Loss Contingencies [Roll Forward]    
Accrual for Environmental Loss Contingencies, Beginning Balance $ 81.0 $ 64.7
Increase Due to CMA Asset Acquisition 22.9  
Additions 8.4 26.5
Payments/Reductions (9.9) (10.2)
Accrual for Environmental Loss Contingencies, Ending Balance 102.4 81.0
The Connecticut Light and Power Company    
Accrual for Environmental Loss Contingencies [Roll Forward]    
Accrual for Environmental Loss Contingencies, Beginning Balance 11.4 5.4
Increase Due to CMA Asset Acquisition 0.0  
Additions 4.2 7.0
Payments/Reductions (3.3) (1.0)
Accrual for Environmental Loss Contingencies, Ending Balance 12.3 11.4
NSTAR Electric Company    
Accrual for Environmental Loss Contingencies [Roll Forward]    
Accrual for Environmental Loss Contingencies, Beginning Balance 8.0 10.9
Increase Due to CMA Asset Acquisition 0.0  
Additions 0.7 0.5
Payments/Reductions (4.0) (3.4)
Accrual for Environmental Loss Contingencies, Ending Balance 4.7 8.0
Public Service Company of New Hampshire    
Accrual for Environmental Loss Contingencies [Roll Forward]    
Accrual for Environmental Loss Contingencies, Beginning Balance 7.5 5.4
Increase Due to CMA Asset Acquisition 0.0  
Additions 0.0 2.8
Payments/Reductions (0.4) (0.7)
Accrual for Environmental Loss Contingencies, Ending Balance $ 7.1 $ 7.5