FAIR VALUE OF FINANCIAL INSTRUMENTS (Details) (USD $)	Jun. 30, 2012	Dec. 31, 2011
Fair Value Balance Sheet Grouping Financial Statement Captions [ Line Items]
Preferred Stock Not Subject to Mandatory Redemption	$ 155,568,000	$ 116,200,000
Rate Reduction Bonds	160,133,000	112,260,000
Carrying Reported Amount Fair Value Disclosure [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [ Line Items]
Preferred Stock Not Subject to Mandatory Redemption	155,600,000	116,200,000
Rate Reduction Bonds	160,100,000	112,300,000
Other Long Term Debt	7,619,700,000	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	153,100,000	105,100,000
Rate Reduction Bonds	163,600,000	116,800,000
Other Long Term Debt	8,302,700,000	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]
Preferred Stock Not Subject to Mandatory Redemption	116,200,000	116,200,000
Other Long Term Debt	2,584,000,000	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	110,100,000	105,100,000
Other Long Term Debt	3,029,300,000	2,987,100,000
Public Service Company Of New Hampshire [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [ Line Items]
Rate Reduction Bonds	57,742,000	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]
Rate Reduction Bonds	57,700,000	85,400,000
Other Long Term Debt	997,800,000	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	59,200,000	88,800,000
Other Long Term Debt	1,089,000,000	1,075,200,000
Western Massachusetts Electric Company [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [ Line Items]
Rate Reduction Bonds	18,079,000	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]
Rate Reduction Bonds	18,100,000	26,900,000
Other Long Term Debt	499,600,000	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	18,600,000	28,100,000
Other Long Term Debt	550,100,000	539,800,000
NSTAR Electric Company [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [ Line Items]
Preferred Stock Not Subject to Mandatory Redemption	43,000,000	43,000,000
Rate Reduction Bonds	84,312,000	127,860,000
NSTAR Electric Company [Member] | Carrying Reported Amount Fair Value Disclosure [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [ Line Items]
Preferred Stock Not Subject to Mandatory Redemption	43,000,000	43,000,000
Rate Reduction Bonds	84,300,000	127,900,000
Other Long Term Debt	1,603,400,000	1,605,100,000
NSTAR 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	43,000,000	41,200,000
Rate Reduction Bonds	85,800,000	131,200,000
Other Long Term Debt	$ 1,842,000,000	$ 1,816,100,000