EMPLOYEE BENEFITS - Pension Plan Narrative (Details) - USD ($) $ in Thousands		12 Months Ended
	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Contribution Plan Disclosure [Line Items]
Liability	$ 962,510	$ 1,370,245	$ 962,510
Expected long-term rate of return		7.00%	7.00%
Compensation rate	4.00%		4.00%
CL&P
Defined Contribution Plan Disclosure [Line Items]
Liability	$ 282,771	$ 391,159	$ 282,771
PSNH
Defined Contribution Plan Disclosure [Line Items]
Liability	124,457	157,638	124,457
Pension Plan
Defined Contribution Plan Disclosure [Line Items]
Increase in liability due to amendment in pension plan	465,000	813,100
Funded status of plan	(946,100)	(1,353,100)	$ (946,100)
Expected long-term rate of return			8.25%	8.25%
Estimated future employer contributions in next fiscal year		105,000
Estimated future contributions in the next twelve months by other subsidiaries		57,300
Pension Plan | CL&P
Defined Contribution Plan Disclosure [Line Items]
Funded status of plan	(242,000)	(345,100)	$ (242,000)
Estimated future employer contributions in next fiscal year		23,200
Pension Plan | PSNH
Defined Contribution Plan Disclosure [Line Items]
Funded status of plan	(104,100)	(141,000)	(104,100)
Estimated future employer contributions in next fiscal year		19,500
Other Postretirement Benefits Plan
Defined Contribution Plan Disclosure [Line Items]
Increase in liability due to amendment in pension plan		88,600	88,000
Funded status of plan	8,100	36,900	$ 8,100
Expected long-term rate of return			8.25%	8.25%
Estimated future employer contributions in next fiscal year		2,900
Other Postretirement Benefits Plan | CL&P
Defined Contribution Plan Disclosure [Line Items]
Funded status of plan	(41,100)	(46,400)	$ (41,100)
Other Postretirement Benefits Plan | PSNH
Defined Contribution Plan Disclosure [Line Items]
Funded status of plan	(20,700)	(17,000)	(20,700)
Other Current Liabilities
Defined Contribution Plan Disclosure [Line Items]
Funded status of plan	8,900	8,700	8,900
Other Long-Term Assets | Other Postretirement Benefits Plan
Defined Contribution Plan Disclosure [Line Items]
Prepaid asset	33,400	62,700	33,400
Accrued Pension, SERP and PBOP | Other Postretirement Benefits Plan
Defined Contribution Plan Disclosure [Line Items]
Liability	$ 25,300	$ 25,800	$ 25,300
Minimum | Pension Plan
Defined Contribution Plan Disclosure [Line Items]
Expected long-term rate of return		8.25%
Compensation rate	3.50%	3.50%	3.50%
Minimum | Other Postretirement Benefits Plan
Defined Contribution Plan Disclosure [Line Items]
Expected long-term rate of return		8.25%
Maximum | Pension Plan
Defined Contribution Plan Disclosure [Line Items]
Compensation rate	4.00%	4.00%	4.00%
Pre-65 Plan | Minimum | Other Postretirement Benefits Plan
Defined Contribution Plan Disclosure [Line Items]
Health care trend rate		3.50%
Pre-65 Plan | Minimum | Other Postretirement Benefits Plan | Aquarion
Defined Contribution Plan Disclosure [Line Items]
Health care trend rate		3.50%
Pre-65 Plan | Maximum | Other Postretirement Benefits Plan
Defined Contribution Plan Disclosure [Line Items]
Health care trend rate		6.50%
Pre-65 Plan | Maximum | Other Postretirement Benefits Plan | Aquarion
Defined Contribution Plan Disclosure [Line Items]
Health care trend rate		6.75%
Post-65 Plan | Minimum | Other Postretirement Benefits Plan
Defined Contribution Plan Disclosure [Line Items]
Health care trend rate		3.50%
Post-65 Plan | Minimum | Other Postretirement Benefits Plan | Aquarion
Defined Contribution Plan Disclosure [Line Items]
Health care trend rate			7.00%
Post-65 Plan | Maximum | Other Postretirement Benefits Plan
Defined Contribution Plan Disclosure [Line Items]
Health care trend rate		5.00%
Eversource Service Pension and PBOP Plans
Defined Contribution Plan Disclosure [Line Items]
Assumed rate of return		8.25%
Eversource PBOP Plan | Other Postretirement Benefits Plan | Equity Securities
Defined Contribution Plan Disclosure [Line Items]
Target Asset Allocation		70.00%
Eversource PBOP Plan | Other Postretirement Benefits Plan | Fixed-Income Securities
Defined Contribution Plan Disclosure [Line Items]
Target Asset Allocation		30.00%
Aquarion Plans
Defined Contribution Plan Disclosure [Line Items]
Assumed rate of return		7.00%
Aquarion Pension Plan | Pension Plan | Equity Securities
Defined Contribution Plan Disclosure [Line Items]
Target Asset Allocation		54.00%
Aquarion Pension Plan | Pension Plan | Debt Securities
Defined Contribution Plan Disclosure [Line Items]
Target Asset Allocation		36.00%
Aquarion Pension Plan | Pension Plan | Other Securities
Defined Contribution Plan Disclosure [Line Items]
Target Asset Allocation		10.00%
Aquarion PBOP Plan | Other Postretirement Benefits Plan | Equity Securities
Defined Contribution Plan Disclosure [Line Items]
Target Asset Allocation		54.00%
Aquarion PBOP Plan | Other Postretirement Benefits Plan | Debt Securities
Defined Contribution Plan Disclosure [Line Items]
Target Asset Allocation		41.00%
Aquarion PBOP Plan | Other Postretirement Benefits Plan | Other Securities
Defined Contribution Plan Disclosure [Line Items]
Target Asset Allocation		5.00%