FAIR VALUE OF FINANCIAL INSTRUMENTS (Details) (USD $)	Sep. 30, 2011	Dec. 31, 2010
Fair Value Balance Sheet Grouping Financial Statement Captions Line Items
Preferred Stock Not Subject to Mandatory Redemption	$ 116,200,000	$ 116,200,000
Rate Reduction Bonds	130,374,000	181,572,000
The Connecticut Light and Power Company [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions Line Items
Preferred Stock Not Subject to Mandatory Redemption	116,200,000	116,200,000
The Connecticut Light and Power Company [Member] \| Carrying Reported Amount Fair Value Disclosure [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions Line Items
Other Long Term Debt	2,587,800,000	2,587,500,000
The Connecticut Light and Power Company [Member] \| Estimate Of Fair Value Fair Value Disclosure [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions Line Items
Preferred Stock Not Subject to Mandatory Redemption	102,000,000	93,700,000
Rate Reduction Bonds	0	0
Other Long Term Debt	2,977,800,000	2,816,700,000
Public Service Company of New Hampshire [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions Line Items
Rate Reduction Bonds	99,366,000	138,247,000
Public Service Company of New Hampshire [Member] \| Carrying Reported Amount Fair Value Disclosure [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions Line Items
Other Long Term Debt	999,500,000	837,300,000
Public Service Company of New Hampshire [Member] \| Estimate Of Fair Value Fair Value Disclosure [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions Line Items
Preferred Stock Not Subject to Mandatory Redemption	0	0
Rate Reduction Bonds	103,300,000	146,900,000
Other Long Term Debt	1,069,300,000	871,400,000
Western Massachusetts Electric Company [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions Line Items
Rate Reduction Bonds	31,007,000	43,325,000
Western Massachusetts Electric Company [Member] \| Carrying Reported Amount Fair Value Disclosure [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions Line Items
Other Long Term Debt	501,000,000	401,000,000
Western Massachusetts Electric Company [Member] \| Estimate Of Fair Value Fair Value Disclosure [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions Line Items
Preferred Stock Not Subject to Mandatory Redemption	0	0
Rate Reduction Bonds	32,600,000	46,400,000
Other Long Term Debt	535,100,000	417,000,000
Carrying Reported Amount Fair Value Disclosure [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions Line Items
Other Long Term Debt	4,950,600,000	4,692,400,000
Estimate Of Fair Value Fair Value Disclosure [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions Line Items
Preferred Stock Not Subject to Mandatory Redemption	102,000,000	93,700,000
Rate Reduction Bonds	135,900,000	193,300,000
Other Long Term Debt	$ 5,512,300,000	$ 5,043,800,000

- Definition

The carrying value (book value) of an entity's issued and outstanding stock which is not included within permanent equity in Stockholders Equity. Temporary equity is a security with redemption features that are outside the control of the issuer, is not classified as an asset or liability in conformity with GAAP, and is not mandatorily redeemable. Includes any type of security that is redeemable at a fixed or determinable price or on a fixed or determinable date or dates, is redeemable at the option of the holder, or has conditions for redemption which are not solely within the control of the issuer. If convertible, the issuer does not control the actions or events necessary to issue the maximum number of shares that could be required to be delivered under the conversion option if the holder exercises the option to convert the stock to another class of equity. If the security is a warrant or a rights issue, the warrant or rights issue is considered to be temporary equity if the issuer cannot demonstrate that it would be able to deliver upon the exercise of the option by the holder in all cases. Includes stock with a put option held by an ESOP and stock redeemable by a holder only in the event of a change in control of the issuer.

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