FAIR VALUE OF FINANCIAL INSTRUMENTS (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Temporary Equity, Carrying Amount, Attributable to Noncontrolling Interest	$ 155,568,000	$ 155,568,000
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	155,600,000
Other Long Term Debt	8,847,700,000	8,851,600,000
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	155,100,000	153,600,000
Other Long Term Debt	9,553,100,000	9,451,200,000
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	2,842,100,000	2,842,000,000
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	113,000,000	112,000,000
Other Long Term Debt	3,260,400,000	3,214,500,000
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
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
Other Long Term Debt	1,797,400,000	1,797,400,000
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	42,100,000	41,600,000
Other Long Term Debt	2,022,100,000	1,993,500,000
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	1,076,300,000	1,076,300,000
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]
Other Long Term Debt	1,156,400,000	1,137,900,000
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	628,200,000	628,500,000
Western Massachusetts Electric Company [Member] | Estimate Of Fair Value Fair Value Disclosure [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Other Long Term Debt	$ 674,400,000	$ 689,400,000