MERGER OF NU AND NSTAR (Details) (USD $) In Millions, except Share data, unless otherwise specified	0 Months Ended	12 Months Ended
	Apr. 09, 2012	Dec. 31, 2012	Dec. 31, 2014	Dec. 31, 2013
Business Combination Consideration Transferred [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	$ 5,005.0
Business Combination Consideration Transferred Other	33.0
Total Purchase Price, Business Combination Consideration Transferred	5,038.0
Business Acquisition, Number of Shares Awarded Vested Immediately Prior to Merger	109,000
Business Combination Recognized Identifiable Assets Acquired Goodwill And Liabilities Assumed Less Noncontrolling Interest [Abstract]
Goodwill Acquired During Period	3,232.0	3,200.0
Business Acquisition, Pro Forma Information [Abstract]
Business Acquisition, Pro Forma Revenue		7,004.0
Business Acquisition, Pro Forma Net Income		630.0
Business Acquisition, Pro Forma Earnings Per Share, Basic		$ 2.00
Business Acquisition, Pro Forma Earnings Per Share, Diluted		$ 1.99
Customer Rate Credits for Business Combination		46.0
Storm Cost Deferral Reduction		40.0
Establishment of Energy Efficiency Fund		15.0
Total Pre-Tax Settlement Agreement Impacts		101.0
Business Combination After Tax Transaction Costs		32.0
After Tax Settlement Costs Incurred		60.0
After-Tax Costs Transaction and Other Non Recurring Costs Excluded from Pro Forma Net Income		92.0
Business Combination, Description
Business Acquisition, Percentage of Voting Interests Acquired	100.00%
Common Stock, Shares, Outstanding			316,983,337	315,273,559
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. 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.
The Connecticut Light And Power Company [Member]
Business Acquisition, Pro Forma Information [Abstract]
Customer Rate Credits for Business Combination		25.0
Storm Cost Deferral Reduction		40.0
Total Pre-Tax Settlement Agreement Impacts		65.0
NSTAR Electric Company [Member]
Business Acquisition, Pro Forma Information [Abstract]
Customer Rate Credits for Business Combination		15.0
Total Pre-Tax Settlement Agreement Impacts		15.0
Western Massachusetts Electric Company [Member]
Business Acquisition, Pro Forma Information [Abstract]
Customer Rate Credits for Business Combination		3.0
Total Pre-Tax Settlement Agreement Impacts		$ 3.0