EMPLOYEE BENEFITS - Pension Plan Narrative (Details) - USD ($) $ in Thousands		12 Months Ended
	Dec. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Contribution Plan Disclosure [Line Items]
Liability	$ 1,653,788	$ 1,653,788	$ 1,370,245
Expected long-term rate of return		7.00%	7.00%
The Connecticut Light and Power Company
Defined Contribution Plan Disclosure [Line Items]
Liability	478,325	$ 478,325	$ 391,159
Public Service Company of New Hampshire
Defined Contribution Plan Disclosure [Line Items]
Liability	184,715	184,715	157,638
Pension Plan
Defined Contribution Plan Disclosure [Line Items]
Increase in liability due to amendment in pension plan	603,000		813,100
Funded status of plan	(1,636,100)	$ (1,636,100)	$ (1,353,100)
Expected long-term rate of return		8.25%	8.25%	8.25%
Estimated future employer contributions in next fiscal year	130,000	$ 130,000
Estimated future contributions in the next twelve months by other subsidiaries		51,100
Pension Plan | The Connecticut Light and Power Company
Defined Contribution Plan Disclosure [Line Items]
Funded status of plan	(434,200)	(434,200)	$ (345,100)
Estimated future employer contributions in next fiscal year	78,900	78,900
Pension Plan | Public Service Company of New Hampshire
Defined Contribution Plan Disclosure [Line Items]
Funded status of plan	(155,000)	(155,000)	(141,000)
Other Postretirement Benefits Plan
Defined Contribution Plan Disclosure [Line Items]
Increase in liability due to amendment in pension plan		68,300	88,600
Funded status of plan	10,200	$ 10,200	$ 36,900
Expected long-term rate of return		8.25%	8.25%	8.25%
Estimated future employer contributions in next fiscal year	2,800	$ 2,800
Other Postretirement Benefits Plan | The Connecticut Light and Power Company
Defined Contribution Plan Disclosure [Line Items]
Funded status of plan	(44,500)	(44,500)	$ (46,400)
Other Postretirement Benefits Plan | Public Service Company of New Hampshire
Defined Contribution Plan Disclosure [Line Items]
Funded status of plan	(30,100)	(30,100)	(17,000)
Other Current Liabilities
Defined Contribution Plan Disclosure [Line Items]
Funded status of plan	6,800	6,800	8,700
Other Long-Term Assets | Other Postretirement Benefits Plan
Defined Contribution Plan Disclosure [Line Items]
Prepaid asset	34,700	34,700	62,700
Accrued Pension, SERP and PBOP | Other Postretirement Benefits Plan
Defined Contribution Plan Disclosure [Line Items]
Liability	$ 24,500	$ 24,500	$ 25,800
Pre-65 Plan | Other Postretirement Benefits Plan
Defined Contribution Plan Disclosure [Line Items]
Health care trend rate		6.30%
Ultimate rate	5.00%	5.00%
Pre-65 Plan | Minimum | Other Postretirement Benefits Plan | Aquarion
Defined Contribution Plan Disclosure [Line Items]
Health care trend rate		3.50%	3.50%
Pre-65 Plan | Maximum | Other Postretirement Benefits Plan | Aquarion
Defined Contribution Plan Disclosure [Line Items]
Health care trend rate		6.50%	6.80%
Post-65 Plan | Other Postretirement Benefits Plan
Defined Contribution Plan Disclosure [Line Items]
Ultimate rate	3.50%	3.50%
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%	70.00%
Eversource PBOP Plan | Other Postretirement Benefits Plan | Fixed-Income Securities
Defined Contribution Plan Disclosure [Line Items]
Target Asset Allocation	30.00%	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%	54.00%
Aquarion Pension Plan | Pension Plan | Debt Securities
Defined Contribution Plan Disclosure [Line Items]
Target Asset Allocation	36.00%	36.00%
Aquarion Pension Plan | Pension Plan | Other Securities
Defined Contribution Plan Disclosure [Line Items]
Target Asset Allocation	10.00%	10.00%
Aquarion PBOP Plan | Other Postretirement Benefits Plan | Equity Securities
Defined Contribution Plan Disclosure [Line Items]
Target Asset Allocation	54.00%	54.00%
Aquarion PBOP Plan | Other Postretirement Benefits Plan | Debt Securities
Defined Contribution Plan Disclosure [Line Items]
Target Asset Allocation	41.00%	41.00%
Aquarion PBOP Plan | Other Postretirement Benefits Plan | Other Securities
Defined Contribution Plan Disclosure [Line Items]
Target Asset Allocation	5.00%	5.00%