SHARE-BASED PAYMENTS - Eversource Incentve Plan (Details) - $ / shares	3 Months Ended	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Beginning balance (in shares)		729,308
Granted (in shares)		305,340
Shares issued (in shares)		(270,060)
Forfeited (in shares)		(40,318)
Ending balance (in shares)		724,270	729,308
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Beginning balance (in dollars per share)		$ 43.45
Granted (in dollars per share)	$ 42.27	54.67	$ 54.57
Shares issued (in dollars per share)		44.94
Forfeited (in dollars per share)		53.99
Ending balance (in dollars per share)		$ 47.86	$ 43.45
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Beginning balance (in shares)		528,428
Granted (in shares)		222,139
Shares issued (in shares)		(201,826)
Forfeited (in shares)		(25,807)
Ending balance (in shares)		522,934	528,428
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Beginning balance (in dollars per share)		$ 46.30
Granted (in dollars per share)		53.64	$ 55.04	$ 43.40
Shares issued (in dollars per share)		40.93
Forfeited (in dollars per share)		54.48
Ending balance (in dollars per share)		$ 51.09	$ 46.30