MERGER OF NU AND NSTAR (Details) (USD $) Share data in Thousands, except Per Share data, unless otherwise specified	0 Months Ended	12 Months Ended
	Apr. 09, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2013
Business Combination Consideration Transferred [Abstract]
Number of NSTAR Shares Outstanding	103,696
Exchange Ratio	1.312
Business Acquisition, Equity Interest Issued or Issuable, Number of Shares	136,049
Business Acquisition Share Price	$ 36.79
Business Acquisition Cost Of Acquired Entity Equity Interests Issued And Issuable	$ 5,005,000,000
Business Combination Consideration Transferred Other	33,000,000
Total Purchase Price, Business Combination Consideration Transferred	5,038,000,000
Business Acquisition, Number of Shares Awarded Vested Immediately Prior to Merger	109
Business Combination Recognized Identifiable Assets Acquired Goodwill And Liabilities Assumed Less Noncontrolling Interest [Abstract]
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed Current Assets		739,000,000
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed Property Plant And Equipment		5,155,000,000
Goodwill Acquired During Period		3,232,000,000
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed Noncurrent Assets		2,103,000,000
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed Current Liabilities		(1,330,000,000)
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed Noncurrent Liabilities Other		(2,723,000,000)
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed Noncurrent Liabilities Long Term Debt		(2,099,000,000)
Business Combination Recognized Identifiable Assets Acquired Goodwill And Liabilities Assumed Less Noncontrolling Interest		5,038,000,000
BusinessCombinationAcquisitionOfLessThan100PercentNoncontrollingInterestFairValue 0		39,000,000
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%
Business Acquisition, Description of Settlement Agreements		Regulatory Approvals: On February 15, 2012, NU and NSTAR reached comprehensive merger settlement agreements with the Massachusetts Attorney General and the DOER. 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 merger settlement agreement with both the Connecticut Attorney General and the Connecticut Office of Consumer Counsel. 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		300,000,000		3,519,401,000
Goodwill Acquired During Period		3,232,000,000
Goodwill, Ending Balance		3,519,401,000	300,000,000	3,519,401,000
Electric Distribution Member
Business Combination Recognized Identifiable Assets Acquired Goodwill And Liabilities Assumed Less Noncontrolling Interest [Abstract]
Goodwill Acquired During Period		2,500,000,000
Goodwill Roll Forward [Abstract]
Goodwill, Beginning Balance				2,500,000,000
Goodwill Acquired During Period		2,500,000,000
Goodwill, Ending Balance		2,500,000,000		2,500,000,000
Transmission [Member]
Business Combination Recognized Identifiable Assets Acquired Goodwill And Liabilities Assumed Less Noncontrolling Interest [Abstract]
Goodwill Acquired During Period		600,000,000
Goodwill Roll Forward [Abstract]
Goodwill, Beginning Balance				600,000,000
Goodwill Acquired During Period		600,000,000
Goodwill, Ending Balance		600,000,000		600,000,000
Natural Gas Distribution [Member]
Business Combination Recognized Identifiable Assets Acquired Goodwill And Liabilities Assumed Less Noncontrolling Interest [Abstract]
Goodwill Acquired During Period		100,000,000
Goodwill Roll Forward [Abstract]
Goodwill, Beginning Balance		300,000,000		400,000,000
Goodwill Acquired During Period		100,000,000
Goodwill, Ending Balance		400,000,000		400,000,000
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