ASSET RETIREMENT OBLIGATIONS - Schedule of Reconciliation of Beginning and Ending Balance (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance as of Beginning of Year	$ 430.1	$ 426.3
Liabilities Incurred During the Year	1.3	6.6
Liabilities Settled During the Year	(19.0)	(18.2)
Accretion	22.9	26.5
Revisions in Estimated Cash Flows	(8.9)	(11.1)
Balance as of End of Year	426.4	430.1
The Connecticut Light And Power Company [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance as of Beginning of Year	33.8	35.3
Liabilities Incurred During the Year	0.0	0.0
Liabilities Settled During the Year	0.0	0.0
Accretion	2.2	2.2
Revisions in Estimated Cash Flows	0.0	(3.7)
Balance as of End of Year	36.0	33.8
NSTAR Electric Company [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance as of Beginning of Year	35.3	34.3
Liabilities Incurred During the Year	0.0	6.2
Liabilities Settled During the Year	(0.3)	(1.5)
Accretion	1.7	1.8
Revisions in Estimated Cash Flows	0.0	(5.5)
Balance as of End of Year	36.7	35.3
Public Service Company Of New Hampshire [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance as of Beginning of Year	21.6	20.6
Liabilities Incurred During the Year	0.5	0.4
Liabilities Settled During the Year	0.0	0.0
Accretion	1.4	1.3
Revisions in Estimated Cash Flows	0.0	(0.7)
Balance as of End of Year	23.5	21.6
Western Massachusetts Electric Company [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance as of Beginning of Year	5.7	5.9
Liabilities Incurred During the Year	0.0	0.0
Liabilities Settled During the Year	(0.1)	(0.1)
Accretion	0.3	0.4
Revisions in Estimated Cash Flows	0.0	(0.5)
Balance as of End of Year	$ 5.9	$ 5.7