MERGER OF NU AND NSTAR (Details) (USD $)	0 Months Ended	12 Months Ended
	Apr. 09, 2012	Dec. 31, 2012	Dec. 31, 2011
Business Acquisition, Cost of Acquired Entity, Purchase Price [Abstract]
Number of NSTAR Shares Outstanding	103,696,000
Exchange Ratio	1.312
Business Acquisition, Equity Interest Issued or Issuable, Number of Shares	136,049,000
Business Acquisition Share Price	$ 36.79
Business Acquisition Cost Of Acquired Entity Equity Interests Issued And Issuable	$ 5,005,000,000
Business Acquisition, Cost of Acquired Entity, Other Noncash Consideration	33,000,000
Business Acquisition, Cost of Acquired Entity, Purchase Price	5,038,000,000	5,000,000,000
Business Acquisition, Purchase Price Allocation [Abstract]
Business Acquisition, Purchase Price Allocation, Current Assets		700,000,000
Business Acquisition, Purchase Price Allocation, Property, Plant and Equipment		5,100,000,000
Business Acquisition, Purchase Price Allocation, Goodwill Amount		3,200,000,000
Business Acquisition, Purchase Price Allocation, Other Noncurrent Assets		2,100,000,000
Business Acquisition, Purchase Price Allocation, Current Liabilities		(1,300,000,000)
Business Acquisition, Purchase Price Allocation, Other Noncurrent Liabilities		(2,700,000,000)
Business Acquisition, Purchase Price Allocation, Noncurrent Liabilities, Long-term debt		(2,100,000,000)
Total Purchase Price	5,038,000,000	5,000,000,000
Business Acquisition, Pro Forma Information [Abstract]
Business Acquisition, Pro Forma Revenue		7,004,000,000	7,361,000,000
Business Acquisition, Pro Forma Net Income		630,000,000	689,000,000
Business Acquisition, Pro Forma Earnings Per Share, Basic		$ 2.00	$ 2.20
Business Acquisition, Pro Forma Earnings Per Share, Diluted		$ 1.99	$ 2.19
Customer Rate Credits for Business Combination		46,000,000
Storm Cost Deferral Reduction		40,000,000
Establishment of Energy Efficiency Fund		15,000,000
Total Pre-Tax Settlement Agreement Impacts		101,000,000
Business Combination After Tax Transaction Costs		32,000,000	19,000,000
After Tax Settlement Costs Incurred		60,000,000	0
After Tax Costs Transaction and Other Non Recurring Costs Excluded from Pro Forma Net Income		92,000,000	19,000,000
Business Combination, Description
Business Acquisition, Percentage of Voting Interests Acquired	100.00%
Common Stock, Shares, Outstanding	314,000,000
Business Acquisition, Number of Shares Awarded Vested Immediately Prior to Merger	109,000
Business Acquisition, Description of Settlement Agreements		Regulatory Approvals: On February 15, 2012, NU and NSTAR reached comprehensive settlement agreements with the Massachusetts Attorney General and the DOER related to the merger. The Attorney General settlement agreement covered a variety of rate-making and rate design issues, including a base distribution rate freeze through 2015 for NSTAR Electric, NSTAR Gas and WMECO and $15 million, $3 million and $3 million in the form of rate credits to their respective customers. The settlement agreement reached with the DOER covered the same rate-making and rate design issues as the Attorney General's settlement agreement, as well as a variety of matters impacting the advancement of Massachusetts clean energy policy established by the Green Communities Act and Global Warming Solutions Act. On April 4, 2012, the DPU approved the settlement agreements and the merger of NU and NSTAR. On March 13, 2012, NU and NSTAR reached a comprehensive settlement agreement with both the Connecticut Attorney General and the Connecticut Office of Consumer Counsel related to the merger. The settlement agreement covered a variety of matters, including a $25 million rate credit to CL&P customers, a CL&P base distribution rate freeze until December 1, 2014, and the establishment of a $15 million fund for energy efficiency and other initiatives to be disbursed at the direction of the DEEP. In the agreement, CL&P agreed to forego rate recovery of $40 million of the deferred storm restoration costs associated with restoration activities following Tropical Storm Irene and the October 2011 snowstorm. On April 2, 2012, the PURA approved the settlement agreement and the merger of NU and NSTAR.
Goodwill Roll Forward [Abstract]
Goodwill, Beginning Balance		287,591,000
GoodwillAcquiredDuringPeriod		3,200,000,000
Goodwill, Ending Balance		3,519,401,000	287,591,000
Electric Distribution Member
Goodwill Roll Forward [Abstract]
GoodwillAcquiredDuringPeriod		2,500,000,000
Goodwill, Ending Balance		2,500,000,000
Transmission [Member]
Goodwill Roll Forward [Abstract]
GoodwillAcquiredDuringPeriod		600,000,000
Goodwill, Ending Balance		600,000,000
Natural Gas Distribution [Member]
Goodwill Roll Forward [Abstract]
Goodwill, Beginning Balance		300,000,000
GoodwillAcquiredDuringPeriod		100,000,000
Goodwill, Ending Balance		400,000,000
NSTAR [Member]
Business Combination, Description
Revenues From Acquired Company		1,957,800,000
Net Income From Acquired Company		182,900,000
The Connecticut Light And Power Company [Member]
Business Acquisition, Pro Forma Information [Abstract]
Customer Rate Credits for Business Combination		25,000,000
Storm Cost Deferral Reduction		40,000,000
Total Pre-Tax Settlement Agreement Impacts		65,000,000
NSTAR Electric Company [Member]
Business Acquisition, Pro Forma Information [Abstract]
Customer Rate Credits for Business Combination		15,000,000
Total Pre-Tax Settlement Agreement Impacts		15,000,000
Western Massachusetts Electric Company [Member]
Business Acquisition, Pro Forma Information [Abstract]
Customer Rate Credits for Business Combination		3,000,000
Total Pre-Tax Settlement Agreement Impacts		$ 3,000,000