EMPLOYEE BENEFITS - Pension Plan Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Defined Contribution Plan Disclosure [Line Items]
Liability	$ 242,463	$ 1,653,788
Expected long-term rate of return	7.00%	7.00%
The Connecticut Light and Power Company
Defined Contribution Plan Disclosure [Line Items]
Liability	$ 26,820	$ 478,325
Public Service Company of New Hampshire
Defined Contribution Plan Disclosure [Line Items]
Liability	30,184	184,715
Pension Plan
Defined Contribution Plan Disclosure [Line Items]
Increase in liability due to amendment in pension plan	286,800	603,000
Funded status of plan	$ (234,200)	$ (1,636,100)
Expected long-term rate of return	8.25%	8.25%	8.25%
Pension Plan | The Connecticut Light and Power Company
Defined Contribution Plan Disclosure [Line Items]
Funded status of plan	$ (7,100)	$ (434,200)
Pension Plan | Public Service Company of New Hampshire
Defined Contribution Plan Disclosure [Line Items]
Funded status of plan	(26,400)	(155,000)
Other Postretirement Benefits Plan
Defined Contribution Plan Disclosure [Line Items]
Increase in liability due to amendment in pension plan	29,800	68,300
Funded status of plan	$ 254,000	$ 10,200
Expected long-term rate of return	8.25%	8.25%	8.25%
Estimated future employer contributions in next fiscal year	$ 2,400
Other Postretirement Benefits Plan | The Connecticut Light and Power Company
Defined Contribution Plan Disclosure [Line Items]
Funded status of plan	(19,800)	$ (44,500)
Other Postretirement Benefits Plan | Public Service Company of New Hampshire
Defined Contribution Plan Disclosure [Line Items]
Funded status of plan	(4,300)	(30,100)
Other Current Liabilities
Defined Contribution Plan Disclosure [Line Items]
Funded status of plan	9,700	6,800
Other Long-Term Assets | Other Postretirement Benefits Plan
Defined Contribution Plan Disclosure [Line Items]
Prepaid asset	272,000	34,700
Accrued Pension, SERP and PBOP | Other Postretirement Benefits Plan
Defined Contribution Plan Disclosure [Line Items]
Liability	18,000	$ 24,500
Minimum | Pension Plan
Defined Contribution Plan Disclosure [Line Items]
Estimated future employer contributions in next fiscal year	100,000
Maximum | Pension Plan
Defined Contribution Plan Disclosure [Line Items]
Estimated future employer contributions in next fiscal year	$ 175,000
Pre-65 Plan | Other Postretirement Benefits Plan
Defined Contribution Plan Disclosure [Line Items]
Health care trend rate	6.50%
Ultimate rate	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	6.20%	6.50%
Post-65 Plan | Other Postretirement Benefits Plan
Defined Contribution Plan Disclosure [Line Items]
Ultimate rate	3.50%
Eversource Service Pension and PBOP Plans
Defined Contribution Plan Disclosure [Line Items]
Assumed rate of return	8.25%
Aquarion Plans
Defined Contribution Plan Disclosure [Line Items]
Assumed rate of return	7.00%