SHARE-BASED PAYMENTS - Eversource Incentve Plan (Details) - $ / shares	3 Months Ended	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Beginning balance (in shares)		724,270
Granted (in shares)		299,285
Shares issued (in shares)		(289,635)
Forfeited (in shares)		(16,881)
Ending balance (in shares)		717,039	724,270
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)		$ 47.86
Granted (in dollars per share)	$ 54.57	55.97	$ 54.67
Shares issued (in dollars per share)		52.26
Forfeited (in dollars per share)		55.60
Ending balance (in dollars per share)		$ 49.29	$ 47.86
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Beginning balance (in shares)		522,934
Granted (in shares)		180,032
Shares issued (in shares)		(173,914)
Forfeited (in shares)		(18,487)
Ending balance (in shares)		510,565	522,934
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)		$ 51.09
Granted (in dollars per share)		55.70	$ 53.64	$ 55.04
Shares issued (in dollars per share)		43.48
Forfeited (in dollars per share)		47.06
Ending balance (in dollars per share)		$ 55.45	$ 51.09