EMPLOYEE BENEFITS - Pension Plan Narrative (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Aug. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Contribution Plan Disclosure [Line Items]
Increase in liability due to amendment in pension plan			$ 390,000
Liability			1,228,091	$ 1,141,514
Decrease in discount rate resulting in increase in liability			64,000
Pension Plan
Defined Contribution Plan Disclosure [Line Items]
Increase in liability due to amendment in pension plan				177,000
Change in liability due to change in mortality rate				32,000
Funded status of plan			$ (1,197,000)	(1,166,300)
Change in pension expense due to change in discount rate methodology for calculating interest and service costs				46,000
Increase in liability due to decrease in discount rate				9,000
Assumed rate of return			8.25%
Other Postretirement Benefits Plan
Defined Contribution Plan Disclosure [Line Items]
Funded status of plan			$ (26,400)	5,800
Change in pension expense due to change in discount rate methodology for calculating interest and service costs				10,000
Accumulated benefit obligation	$ 244,000
Increase in liability due to decrease in discount rate	$ 75,000
Change in mortality assumptions and other demographic assumptions			$ 52,000
Impact of plan amendment on annual expense				$ 10,000
Assumptions used in calculating net periodic benefit cost, health care cost rate (in percentage)				6.25%	6.50%
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			$ 354,469	$ 300,208
The Connecticut Light And Power Company | Pension Plan
Defined Contribution Plan Disclosure [Line Items]
Funded status of plan			(312,200)	(264,700)
The Connecticut Light And Power Company | Other Postretirement Benefits Plan
Defined Contribution Plan Disclosure [Line Items]
Funded status of plan			(42,500)	(35,800)
Public Service Company Of New Hampshire
Defined Contribution Plan Disclosure [Line Items]
Liability			124,639	94,652
Public Service Company Of New Hampshire | Pension Plan
Defined Contribution Plan Disclosure [Line Items]
Funded status of plan			(102,700)	(78,200)
Public Service Company Of New Hampshire | Other Postretirement Benefits Plan
Defined Contribution Plan Disclosure [Line Items]
Funded status of plan			(22,100)	(16,500)
NSTAR Electric Company
Defined Contribution Plan Disclosure [Line Items]
Liability			89,995	128,751
NSTAR Electric Company | Pension Plan
Defined Contribution Plan Disclosure [Line Items]
Funded status of plan			(90,200)	(129,000)
NSTAR Electric Company | Other Postretirement Benefits Plan
Defined Contribution Plan Disclosure [Line Items]
Funded status of plan			126,900	91,600
Other Current Liabilities
Defined Contribution Plan Disclosure [Line Items]
Funded status of plan				$ 8,400	$ 24,800
Other Noncurrent Assets
Defined Contribution Plan Disclosure [Line Items]
Prepaid asset			13,000
Accrued Pension, SERP and PBOP
Defined Contribution Plan Disclosure [Line Items]
Liability			$ 39,500
Scenario, Forecast | Pension Plan
Defined Contribution Plan Disclosure [Line Items]
Estimated future employer contributions in next fiscal year		$ 180,000
Estimated future contributions in the next twelve months by other subsidiaries		92,000
Scenario, Forecast | Other Postretirement Benefits Plan
Defined Contribution Plan Disclosure [Line Items]
Estimated future employer contributions in next fiscal year		10,000
Scenario, Forecast | The Connecticut Light And Power Company | Pension Plan
Defined Contribution Plan Disclosure [Line Items]
Estimated future employer contributions in next fiscal year		82,000
Scenario, Forecast | Public Service Company Of New Hampshire | Pension Plan
Defined Contribution Plan Disclosure [Line Items]
Estimated future employer contributions in next fiscal year		6,000
Scenario, Forecast | NSTAR Electric Company | Other Postretirement Benefits Plan
Defined Contribution Plan Disclosure [Line Items]
Estimated future employer contributions in next fiscal year		$ 5,000