FAIR VALUE OF FINANCIAL INSTRUMENTS (Details) (USD $)	Sep. 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	101,347,000	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	101,300,000	112,300,000
Other Long Term Debt	7,612,300,000	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	154,000,000	105,100,000
Rate Reduction Bonds	103,000,000	116,800,000
Other Long Term Debt	8,378,500,000	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	2,584,200,000	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	112,000,000	105,100,000
Other Long Term Debt	3,063,000,000	2,987,100,000
Public Service Company Of New Hampshire [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Rate Reduction Bonds	44,103,000	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	44,100,000	85,400,000
Other Long Term Debt	997,900,000	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	44,600,000	88,800,000
Other Long Term Debt	1,098,000,000	1,075,200,000
Western Massachusetts Electric Company [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Rate Reduction Bonds	13,751,000	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	13,800,000	26,900,000
Other Long Term Debt	499,700,000	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	14,000,000	28,100,000
Other Long Term Debt	564,300,000	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	43,493,000	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	43,500,000	127,900,000
Other Long Term Debt	1,603,600,000	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	42,000,000	41,200,000
Rate Reduction Bonds	44,300,000	131,200,000
Other Long Term Debt	$ 1,846,400,000	$ 1,816,100,000