ASSET RETIREMENT OBLIGATIONS - Schedule of Reconciliation of Beginning and Ending Balance (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance as of Beginning of Year	$ 426.4	$ 430.1
Liabilities Incurred During the Year	0.2	1.3
Liabilities Settled During the Year	(19.3)	(19.0)
Accretion	26.3	22.9
Revisions in Estimated Cash Flows	(14.5)	(8.9)
Balance as of End of Year	419.1	426.4
The Connecticut Light And Power Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance as of Beginning of Year	36.0	33.8
Liabilities Incurred During the Year	0.1	0.0
Liabilities Settled During the Year	(1.0)	0.0
Accretion	2.3	2.2
Revisions in Estimated Cash Flows	(5.9)	0.0
Balance as of End of Year	31.5	36.0
NSTAR Electric Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance as of Beginning of Year	42.6	41.0
Liabilities Incurred During the Year	0.1	0.0
Liabilities Settled During the Year	(0.2)	(0.4)
Accretion	2.1	2.0
Revisions in Estimated Cash Flows	0.0	0.0
Balance as of End of Year	44.6	42.6
Public Service Company Of New Hampshire
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance as of Beginning of Year	23.5	21.6
Liabilities Incurred During the Year	0.0	0.5
Liabilities Settled During the Year	0.0	0.0
Accretion	1.5	1.4
Revisions in Estimated Cash Flows	0.0	0.0
Balance as of End of Year	$ 25.0	$ 23.5