EMPLOYEE BENEFITS - Pension Plan Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Contribution Plan Disclosure [Line Items]
Liability		$ 962,510	$ 1,228,091
Pension Plan
Defined Contribution Plan Disclosure [Line Items]
Increase in liability due to amendment in pension plan		465,000	390,000
Funded status of plan		$ (946,100)	(1,197,000)
Assumed rate of return		8.25%
Other Postretirement Benefits Plan
Defined Contribution Plan Disclosure [Line Items]
Funded status of plan		$ 8,100	(26,400)
Defined Benefit Plan, Increase in Discount Rate Resulting in Decrease in Liability		$ 88,000
Decrease in discount rate resulting in increase in liability			64,000
Taxable assets in equity securities (in percentage)		70.00%
Taxable assets in fixed income securities		30.00%
The Connecticut Light And Power Company
Defined Contribution Plan Disclosure [Line Items]
Liability		$ 282,771	354,469
The Connecticut Light And Power Company | Pension Plan
Defined Contribution Plan Disclosure [Line Items]
Funded status of plan		(242,000)	(312,200)
The Connecticut Light And Power Company | Other Postretirement Benefits Plan
Defined Contribution Plan Disclosure [Line Items]
Funded status of plan		(41,100)	(42,500)
Public Service Company Of New Hampshire
Defined Contribution Plan Disclosure [Line Items]
Liability		124,457	124,639
Public Service Company Of New Hampshire | Pension Plan
Defined Contribution Plan Disclosure [Line Items]
Funded status of plan		(104,100)	(102,700)
Public Service Company Of New Hampshire | Other Postretirement Benefits Plan
Defined Contribution Plan Disclosure [Line Items]
Funded status of plan		(20,700)	(22,100)
NSTAR Electric Company
Defined Contribution Plan Disclosure [Line Items]
Liability		14,145	89,995
NSTAR Electric Company | Pension Plan
Defined Contribution Plan Disclosure [Line Items]
Funded status of plan		(14,400)	(90,200)
NSTAR Electric Company | Other Postretirement Benefits Plan
Defined Contribution Plan Disclosure [Line Items]
Funded status of plan		132,800	126,900
Other Current Liabilities
Defined Contribution Plan Disclosure [Line Items]
Funded status of plan			8,900	$ 8,400
Other Noncurrent Assets | Other Postretirement Benefits Plan
Defined Contribution Plan Disclosure [Line Items]
Prepaid asset		33,400	13,100
Accrued Pension, SERP and PBOP | Other Postretirement Benefits Plan
Defined Contribution Plan Disclosure [Line Items]
Liability		$ 25,300	$ 39,500
Scenario, Forecast | Pension Plan
Defined Contribution Plan Disclosure [Line Items]
Estimated future employer contributions in next fiscal year	$ 112,000
Estimated future contributions in the next twelve months by other subsidiaries	46,000
Scenario, Forecast | Other Postretirement Benefits Plan
Defined Contribution Plan Disclosure [Line Items]
Estimated future employer contributions in next fiscal year	11,000
Scenario, Forecast | The Connecticut Light And Power Company | Pension Plan
Defined Contribution Plan Disclosure [Line Items]
Estimated future employer contributions in next fiscal year	44,000
Scenario, Forecast | Public Service Company Of New Hampshire | Pension Plan
Defined Contribution Plan Disclosure [Line Items]
Estimated future employer contributions in next fiscal year	10,000
Scenario, Forecast | NSTAR Electric Company | Other Postretirement Benefits Plan
Defined Contribution Plan Disclosure [Line Items]
Estimated future employer contributions in next fiscal year	$ 6,000