ASSET RETIREMENT OBLIGATIONS (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation, Beginning Balance	$ 53.3	$ 50.6
Asset Retirement Obligation, Liabilities Incurred	2.1	0.2
Asset Retirement Obligation, Liabilities Settled	(0.8)	(1.2)
Asset Retirement Obligation, Accretion Expense	3.5	3.3
Asset Retirement Obligation, Revision of Estimate	(1.9)	0.4
Asset Retirement Obligation, Ending Balance	56.2	53.3
The Connecticut Light And Power Company [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation, Beginning Balance	29.3	28.6
Asset Retirement Obligation, Liabilities Incurred	1.7	0.1
Asset Retirement Obligation, Liabilities Settled	(0.8)	(1.2)
Asset Retirement Obligation, Accretion Expense	2.0	1.8
Asset Retirement Obligation, Revision of Estimate	0	0
Asset Retirement Obligation, Ending Balance	32.2	29.3
Public Service Company Of New Hampshire [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation, Beginning Balance	17.6	16.4
Asset Retirement Obligation, Liabilities Incurred	0.2	0
Asset Retirement Obligation, Liabilities Settled	0	0
Asset Retirement Obligation, Accretion Expense	1.1	1.1
Asset Retirement Obligation, Revision of Estimate	(1.9)	0.1
Asset Retirement Obligation, Ending Balance	17.0	17.6
Western Massachusetts Electric Company [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation, Beginning Balance	3.6	3.3
Asset Retirement Obligation, Liabilities Incurred	0.2	0.1
Asset Retirement Obligation, Liabilities Settled	0	0
Asset Retirement Obligation, Accretion Expense	0.2	0.2
Asset Retirement Obligation, Revision of Estimate	0	0
Asset Retirement Obligation, Ending Balance	$ 4.0	$ 3.6