ASSET RETIREMENT OBLIGATIONS (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation, Beginning Balance	$ 56.2	$ 53.3
Liability Assumed Upon Consolidation of CYAPC and YAEC	284.2
Liability Assumed Upon Merger With Nstar	35.9
Asset Retirement Obligation, Liabilities Incurred		2.1
Asset Retirement Obligation, Liabilities Settled	(7.2)	(0.8)
Asset Retirement Obligation, Accretion Expense	20.2	3.5
Asset Retirement Obligation, Revision of Estimate	21.4	(1.9)
Asset Retirement Obligation, Ending Balance	412.2	56.2
The Connecticut Light And Power Company [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation, Beginning Balance	32.2	29.3
Asset Retirement Obligation, Liabilities Incurred		1.7
Asset Retirement Obligation, Liabilities Settled	(0.9)	(0.8)
Asset Retirement Obligation, Accretion Expense	2.0	2.0
Asset Retirement Obligation, Revision of Estimate	0.3
Asset Retirement Obligation, Ending Balance	33.6	32.2
Public Service Company Of New Hampshire [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation, Beginning Balance	17.0	17.6
Asset Retirement Obligation, Liabilities Incurred	0.3	0.2
Asset Retirement Obligation, Accretion Expense	1.1	1.1
Asset Retirement Obligation, Revision of Estimate		(1.9)
Asset Retirement Obligation, Ending Balance	18.4	17.0
Western Massachusetts Electric Company [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation, Beginning Balance	4.0	3.6
Asset Retirement Obligation, Liabilities Incurred		0.2
Asset Retirement Obligation, Accretion Expense	0.3	0.2
Asset Retirement Obligation, Ending Balance	4.3	4.0
NSTAR Electric Company [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation, Beginning Balance	27.5	26.2
Asset Retirement Obligation, Liabilities Settled	(1.0)
Asset Retirement Obligation, Accretion Expense	1.5	1.3
Asset Retirement Obligation, Ending Balance	$ 31.4	$ 27.5