SHARE-BASED PAYMENTS - Eversource Incentve Plan (Details) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
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)	717,039
Granted (in shares)	286,315
Shares issued (in shares)	(201,386)
Forfeited (in shares)	(19,603)
Ending balance (in shares)	782,365	717,039
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)	$ 49.29
Granted (in dollars per share)	56.69	$ 55.97	$ 54.67
Shares issued (in dollars per share)	55.35
Forfeited (in dollars per share)	56.78
Ending balance (in dollars per share)	$ 50.25	$ 49.29
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)	510,565
Granted (in shares)	184,355
Shares issued (in shares)	(178,258)
Forfeited (in shares)	(17,098)
Ending balance (in shares)	499,564	510,565
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)	$ 55.45
Granted (in dollars per share)	56.77	$ 55.70	$ 53.64
Shares issued (in dollars per share)	54.98
Forfeited (in dollars per share)	56.18
Ending balance (in dollars per share)	$ 56.08	$ 55.45