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COMMITMENTS AND CONTINGENCIES - Schedule of Reconciliation of Activity in Environmental Reserve (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Accrual for Environmental Loss Contingencies [Roll Forward]
Accrual for Environmental Loss Contingencies, Beginning Balance	$ 102.4	$ 81.0
Increase Due to CMA Asset Acquisition		22.9
Additions	23.4	8.4
Payments/Reductions	(10.4)	(9.9)
Accrual for Environmental Loss Contingencies, Ending Balance	115.4	102.4
The Connecticut Light and Power Company
Accrual for Environmental Loss Contingencies [Roll Forward]
Accrual for Environmental Loss Contingencies, Beginning Balance	12.3	11.4
Increase Due to CMA Asset Acquisition		0.0
Additions	4.4	4.2
Payments/Reductions	(2.8)	(3.3)
Accrual for Environmental Loss Contingencies, Ending Balance	13.9	12.3
NSTAR Electric Company
Accrual for Environmental Loss Contingencies [Roll Forward]
Accrual for Environmental Loss Contingencies, Beginning Balance	4.7	8.0
Increase Due to CMA Asset Acquisition		0.0
Additions	0.0	0.7
Payments/Reductions	(1.4)	(4.0)
Accrual for Environmental Loss Contingencies, Ending Balance	3.3	4.7
Public Service Company of New Hampshire
Accrual for Environmental Loss Contingencies [Roll Forward]
Accrual for Environmental Loss Contingencies, Beginning Balance	7.1	7.5
Increase Due to CMA Asset Acquisition		0.0
Additions	0.0	0.0
Payments/Reductions	(0.8)	(0.4)
Accrual for Environmental Loss Contingencies, Ending Balance	$ 6.3	$ 7.1

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Total costs accrued as of the balance sheet date for environmental loss contingencies.

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Amount of expense for newly incurred environmental loss contingencies. Includes, but is not limited to, remediation expense and interest accretion.

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