FAIR VALUE OF FINANCIAL INSTRUMENTS (Details) (USD $)	Dec. 31, 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	112,260,000	181,572,000
Carrying Reported Amount Fair Value Disclosure [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Other Long Term Debt	4,950,700,000	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	105,100,000	93,700,000
Rate Reduction Bonds	116,800,000	193,300,000
Other Long Term Debt	5,517,000,000	5,043,800,000
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	105,100,000	93,700,000
Rate Reduction Bonds	0	0
Other Long Term Debt	2,987,100,000	2,816,700,000
Public Service Company Of New Hampshire [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Rate Reduction Bonds	85,368,000	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
Rate Reduction Bonds	88,800,000	146,900,000
Other Long Term Debt	1,075,200,000	871,400,000
Western Massachusetts Electric Company [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Rate Reduction Bonds	26,892,000	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	501,100,000	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
Rate Reduction Bonds	28,100,000	46,400,000
Other Long Term Debt	$ 539,800,000	$ 417,000,000