ASSET RETIREMENT OBLIGATIONS - Schedule of Reconciliation of Beginning and Ending Balance (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance as of Beginning of Year	$ 489.5	$ 466.2
Liability Assumed Upon CMA Asset Acquisition	20.1	0.0
Liabilities Incurred During the Year	2.1	30.3
Liabilities Settled During the Year	(21.8)	(21.3)
Accretion	28.9	27.1
Revisions in Estimated Cash Flows	(19.1)	(12.8)
Balance as of End of Year	499.7	489.5
The Connecticut Light and Power Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance as of Beginning of Year	32.0	33.5
Liability Assumed Upon CMA Asset Acquisition	0.0	0.0
Liabilities Incurred During the Year	0.0	0.0
Liabilities Settled During the Year	(0.7)	(3.6)
Accretion	2.1	2.2
Revisions in Estimated Cash Flows	0.0	(0.1)
Balance as of End of Year	33.4	32.0
NSTAR Electric Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance as of Beginning of Year	97.5	72.4
Liability Assumed Upon CMA Asset Acquisition	0.0	0.0
Liabilities Incurred During the Year	2.1	30.3
Liabilities Settled During the Year	(1.0)	0.0
Accretion	4.3	3.5
Revisions in Estimated Cash Flows	(11.1)	(8.7)
Balance as of End of Year	91.8	97.5
Public Service Company of New Hampshire
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance as of Beginning of Year	4.2	4.0
Liability Assumed Upon CMA Asset Acquisition	0.0	0.0
Liabilities Incurred During the Year	0.0	0.0
Liabilities Settled During the Year	0.0	0.0
Accretion	0.2	0.2
Revisions in Estimated Cash Flows	0.0	0.0
Balance as of End of Year	$ 4.4	$ 4.2