FAIR VALUE OF FINANCIAL INSTRUMENTS (Details) (USD $)	Dec. 31, 2012	Dec. 31, 2011
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Temporary Equity, Carrying Amount, Attributable to Noncontrolling Interest	$ 155,568,000	$ 116,200,000
Rate Reduction Bonds	82,139,000	112,260,000
Carrying Reported Amount Fair Value Disclosure [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Temporary Equity, Carrying Amount, Attributable to Noncontrolling Interest	155,600,000	116,200,000
Rate Reduction Bonds	82,100,000	112,300,000
Other Long Term Debt	7,963,500,000	4,950,700,000
Estimate Of Fair Value Fair Value Disclosure [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Temporary Equity, Carrying Amount, Attributable to Noncontrolling Interest	152,200,000	105,100,000
Rate Reduction Bonds	83,000,000	116,800,000
Other Long Term Debt	8,640,700,000	5,517,000,000
The Connecticut Light And Power Company [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Temporary Equity, Carrying Amount, Attributable to Noncontrolling Interest	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]
Temporary Equity, Carrying Amount, Attributable to Noncontrolling Interest	116,200,000	116,200,000
Other Long Term Debt	2,862,800,000	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]
Temporary Equity, Carrying Amount, Attributable to Noncontrolling Interest	110,000,000	105,100,000
Other Long Term Debt	3,295,400,000	2,987,100,000
Public Service Company Of New Hampshire [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Rate Reduction Bonds	29,294,000	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	29,300,000	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	29,600,000	88,800,000
Other Long Term Debt	1,088,000,000	1,075,200,000
Western Massachusetts Electric Company [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Rate Reduction Bonds	9,352,000	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	9,400,000	26,900,000
Other Long Term Debt	605,300,000	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	9,500,000	28,100,000
Other Long Term Debt	660,400,000	539,800,000
NSTAR Electric Company [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Temporary Equity, Carrying Amount, Attributable to Noncontrolling Interest	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]
Temporary Equity, Carrying Amount, Attributable to Noncontrolling Interest	43,000,000	43,000,000
Rate Reduction Bonds	43,500,000	127,900,000
Other Long Term Debt	1,602,600,000	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]
Temporary Equity, Carrying Amount, Attributable to Noncontrolling Interest	42,200,000	41,200,000
Rate Reduction Bonds	43,900,000	131,200,000
Other Long Term Debt	$ 1,818,800,000	$ 1,816,100,000