FAIR VALUE OF FINANCIAL INSTRUMENTS (Details) (USD $)	Mar. 31, 2012	Dec. 31, 2011
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	94,357,000	112,260,000
Carrying Reported Amount Fair Value Disclosure [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [ Line Items]
Other Long Term Debt	5,250,700,000	4,950,700,000
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	106,900,000	105,100,000
Rate Reduction Bonds	97,600,000	116,800,000
Other Long Term Debt	5,744,000,000	5,517,000,000
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	2,587,800,000
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	106,900,000	105,100,000
Other Long Term Debt	2,946,500,000	2,987,100,000
Public Service Company Of New Hampshire [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [ Line Items]
Rate Reduction Bonds	71,905,000	85,368,000
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	999,500,000
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]
Rate Reduction Bonds	74,300,000	88,800,000
Other Long Term Debt	1,056,300,000	1,075,200,000
Western Massachusetts Electric Company [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [ Line Items]
Rate Reduction Bonds	22,452,000	26,892,000
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	501,100,000
Western Massachusetts Electric Company [Member] | Estimate Of Fair Value Fair Value Disclosure [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [ Line Items]
Rate Reduction Bonds	23,300,000	28,100,000
Other Long Term Debt	$ 527,000,000	$ 539,800,000