ASSET RETIREMENT OBLIGATIONS - Schedule of Reconciliation of Beginning and Ending Balance (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance as of Beginning of Year	$ 466.2	$ 419.1
Liabilities Incurred During the Year	30.3	11.3
Liabilities Settled During the Year	(21.3)	(36.6)
Accretion	27.1	25.5
Revisions in Estimated Cash Flows	(12.8)	46.9
Balance as of End of Year	489.5	466.2
CL&P
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance as of Beginning of Year	33.5	31.5
Liabilities Incurred During the Year	0.0	0.0
Liabilities Settled During the Year	(3.6)	0.0
Accretion	2.2	2.0
Revisions in Estimated Cash Flows	(0.1)	0.0
Balance as of End of Year	32.0	33.5
NSTAR Electric
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance as of Beginning of Year	72.4	44.6
Liabilities Incurred During the Year	30.3	11.3
Liabilities Settled During the Year	0.0	0.0
Accretion	3.5	2.2
Revisions in Estimated Cash Flows	(8.7)	14.3
Balance as of End of Year	97.5	72.4
PSNH
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance as of Beginning of Year	4.0	25.0
Liabilities Incurred During the Year	0.0	0.0
Liabilities Settled During the Year	0.0	(21.5)
Accretion	0.2	0.5
Revisions in Estimated Cash Flows	0.0	0.0
Balance as of End of Year	$ 4.2	$ 4.0