FAIR VALUE OF FINANCIAL INSTRUMENTS (Details) (USD $)	Jun. 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	147,252,000	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	2,587,700,000	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	97,600,000	93,700,000
Rate Reduction Bonds	0	0
Other Long Term Debt	2,835,400,000	2,816,700,000
Public Service Company of New Hampshire [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions Line Items
Rate Reduction Bonds	112,195,000	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	839,500,000	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	118,500,000	146,900,000
Other Long Term Debt	884,200,000	871,400,000
Western Massachusetts Electric Company [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions Line Items
Rate Reduction Bonds	35,057,000	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	401,000,000	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	37,200,000	46,400,000
Other Long Term Debt	417,900,000	417,000,000
Carrying Reported Amount Fair Value Disclosure [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions Line Items
Other Long Term Debt	4,694,800,000	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	97,600,000	93,700,000
Rate Reduction Bonds	155,700,000	193,300,000
Other Long Term Debt	$ 5,066,200,000	$ 5,043,800,000