EMPLOYEE BENEFITS - Pension Plan Narrative (Details) - USD ($) $ in Millions	1 Months Ended		12 Months Ended
	Aug. 31, 2016	Aug. 31, 2015	Dec. 31, 2016	Dec. 31, 2015
Pension Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Decrease in liability due to change in discount rate			$ 177.0	$ 267.0
Change in liability due to change in mortality rate			32.0	48.0
Increase in liability due to amendment in pension plan			9.0
Lump sum payout		$ 149.5
Funded status of plan			(1,166.3)	(1,174.7)
Change in pension expense due to change in discount rate methodology for calculating interest and service costs			46.0
Employer contributions			146.2	154.6
Estimated future employer contributions in next fiscal year			175.0
Estimated future contributions in the next twelve months by other subsidiaries			$ 148.0
Assumed rate of return			8.25%
Other Postretirement Benefit Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Amount resulting from revised mortality table and estimated benefits				23.0
Funded status of plan			$ 5.8	(239.2)
Change in pension expense due to change in discount rate methodology for calculating interest and service costs			10.0
Accumulated benefit obligation	$ 244.0
Increase in liability due to decrease in discount rate	$ 75.0
Change in mortality assumptions and other demographic assumptions			52.0
Defined Benefit Plan, Decrease in Liability Due to Decrease in Discount Rate				$ 60.0
Impact of plan amendment on annual expense			$ 10.0
Assumptions used in calculating net periodic benefit cost, health care cost rate (in percentage)			6.25%	6.50%
Increase in service and interest cost expense			$ 4.4
Decrease in service and interest cost			3.4
Employer contributions			12.5	$ 7.9
Estimated future employer contributions in next fiscal year			$ 7.6
Taxable assets in equity securities (in percentage)			70.00%
Taxable assets in fixed income securities			30.00%
The Connecticut Light And Power Company [Member] | Pension Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Funded status of plan			$ (264.7)	(244.1)
Employer contributions			0.4	0.0
Estimated future employer contributions in next fiscal year			2.0
The Connecticut Light And Power Company [Member] | Other Postretirement Benefit Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Funded status of plan			(35.8)	(27.3)
Employer contributions			0.0	0.0
NSTAR Electric Company [Member] | Pension Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Funded status of plan			(112.1)	(116.8)
Employer contributions			28.4	5.0
Estimated future employer contributions in next fiscal year			25.0
NSTAR Electric Company [Member] | Other Postretirement Benefit Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Funded status of plan			95.1	(92.5)
Employer contributions			8.9	4.9
Estimated future employer contributions in next fiscal year			5.0
Public Service Company Of New Hampshire [Member] | Pension Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Funded status of plan			(78.2)	(77.1)
Employer contributions			17.1	1.0
Public Service Company Of New Hampshire [Member] | Other Postretirement Benefit Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Funded status of plan			(16.5)	(12.7)
Employer contributions			0.0	0.0
Western Massachusetts Electric Company [Member] | Pension Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Funded status of plan			(16.9)	(16.8)
Employer contributions			0.0	0.0
Western Massachusetts Electric Company [Member] | Other Postretirement Benefit Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Funded status of plan			(3.5)	(2.7)
Employer contributions			0.0	0.0
Other Current Liabilities [Member]
Defined Contribution Plan Disclosure [Line Items]
Funded status of plan			$ 24.8	$ 6.6