ASSET RETIREMENT OBLIGATIONS - Schedule of Reconciliation of Beginning and Ending Balance (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance as of Beginning of Year	$ 419.1	$ 426.4
Liabilities Incurred During the Year	11.3	0.2
Liabilities Settled During the Year	(36.6)	(19.3)
Accretion	25.5	26.3
Revisions in Estimated Cash Flows	46.9	(14.5)
Balance as of End of Year	466.2	419.1
The Connecticut Light And Power Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance as of Beginning of Year	31.5	36.0
Liabilities Incurred During the Year	0.0	0.1
Liabilities Settled During the Year	0.0	(1.0)
Accretion	2.0	2.3
Revisions in Estimated Cash Flows	0.0	(5.9)
Balance as of End of Year	33.5	31.5
NSTAR Electric Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance as of Beginning of Year	44.6	42.6
Liabilities Incurred During the Year	11.3	0.1
Liabilities Settled During the Year	0.0	(0.2)
Accretion	2.2	2.1
Revisions in Estimated Cash Flows	14.3	0.0
Balance as of End of Year	72.4	44.6
Public Service Company Of New Hampshire
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance as of Beginning of Year	25.0	23.5
Liabilities Incurred During the Year	0.0	0.0
Liabilities Settled During the Year	(21.5)	0.0
Accretion	0.5	1.5
Revisions in Estimated Cash Flows	0.0	0.0
Balance as of End of Year	$ 4.0	$ 25.0