ASSET RETIREMENT OBLIGATIONS - Schedule of Reconciliation of Beginning and Ending Balance (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance as of Beginning of Year	$ 499.7	$ 489.5
Liability Assumed Upon CMA Asset Acquisition	0.0	20.1
Liabilities Incurred During the Year	0.0	2.1
Liabilities Settled During the Year	(23.9)	(21.8)
Accretion	29.4	28.9
Revisions in Estimated Cash Flows	(5.1)	(19.1)
Balance as of End of Year	500.1	499.7
The Connecticut Light and Power Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance as of Beginning of Year	33.4	32.0
Liability Assumed Upon CMA Asset Acquisition	0.0	0.0
Liabilities Incurred During the Year	0.0	0.0
Liabilities Settled During the Year	(0.6)	(0.7)
Accretion	2.2	2.1
Revisions in Estimated Cash Flows	0.0	0.0
Balance as of End of Year	35.0	33.4
NSTAR Electric Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance as of Beginning of Year	91.8	97.5
Liability Assumed Upon CMA Asset Acquisition	0.0	0.0
Liabilities Incurred During the Year	0.0	2.1
Liabilities Settled During the Year	0.0	(1.0)
Accretion	4.0	4.3
Revisions in Estimated Cash Flows	1.7	(11.1)
Balance as of End of Year	97.5	91.8
Public Service Company of New Hampshire
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance as of Beginning of Year	4.4	4.2
Liability Assumed Upon CMA Asset Acquisition	0.0	0.0
Liabilities Incurred During the Year	0.0	0.0
Liabilities Settled During the Year	0.0	0.0
Accretion	0.3	0.2
Revisions in Estimated Cash Flows	0.0	0.0
Balance as of End of Year	$ 4.7	$ 4.4