EMPLOYEE BENEFITS - Schedule of Pension Benefit Obligations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	$ (5,936.5)	$ (5,242.3)
Service Cost	(84.8)	(71.3)	$ (75.0)
Interest Cost	(196.4)	(188.0)	(185.5)
Actuarial Gain/(Loss)	414.9	(548.7)
Benefits Paid - Pension	261.8	243.7
Benefits Paid - Lump Sum	14.2	18.4
Benefits Paid - SERP	6.8	20.4
Employee Transfers	0.0	0.0
Increase due to acquisition of Aquarion	0.0	(168.7)
Benefit Obligation as of End of Year	(5,520.0)	(5,936.5)	(5,242.3)
Change in Pension Plan Assets:
Fair Value of Pension Plan Assets as of Beginning of Year	4,739.5	4,076.0
Employer Contributions	185.6	235.2
Actual Return on Pension Plan Assets	(75.2)	589.7
Benefits Paid - Pension	(261.8)	(243.7)
Benefits Paid - Lump Sum	(14.2)	(18.4)
Employee Transfers	0.0	0.0
Increase due to acquisition of Aquarion	0.0	100.7
Fair Value of Pension Plan Assets as of End of Year	4,573.9	4,739.5	4,076.0
Funded Status as of December 31st	(946.1)	(1,197.0)
Other Postretirement Benefits Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	(948.6)	(810.0)
Service Cost	(10.0)	(9.5)	(12.2)
Interest Cost	(30.7)	(27.1)	(32.9)
Actuarial Gain/(Loss)	102.5	(81.8)
Benefits Paid - Pension	45.3	41.5
Employee Transfers	0.0	0.0
Increase due to acquisition of Aquarion	0.0	(61.7)
Benefit Obligation as of End of Year	(841.5)	(948.6)	(810.0)
Change in Pension Plan Assets:
Fair Value of Pension Plan Assets as of Beginning of Year	922.2	815.8
Employer Contributions	9.3	7.6
Actual Return on Pension Plan Assets	(36.6)	118.0
Benefits Paid - Pension	(45.3)	(41.5)
Employee Transfers	0.0	0.0
Increase due to acquisition of Aquarion	0.0	22.3
Fair Value of Pension Plan Assets as of End of Year	849.6	922.2	815.8
Funded Status as of December 31st	8.1	(26.4)
The Connecticut Light And Power Company | Pension Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	(1,275.2)	(1,170.2)
Service Cost	(21.4)	(18.5)	(18.8)
Interest Cost	(41.8)	(41.6)	(41.6)
Actuarial Gain/(Loss)	106.1	(116.9)
Benefits Paid - Pension	59.6	63.5
Benefits Paid - Lump Sum	0.0	0.0
Benefits Paid - SERP	0.3	0.3
Employee Transfers	12.0	8.2
Increase due to acquisition of Aquarion	0.0	0.0
Benefit Obligation as of End of Year	(1,160.4)	(1,275.2)	(1,170.2)
Change in Pension Plan Assets:
Fair Value of Pension Plan Assets as of Beginning of Year	963.0	905.5
Employer Contributions	41.2	2.5
Actual Return on Pension Plan Assets	(14.2)	126.7
Benefits Paid - Pension	(59.6)	(63.5)
Benefits Paid - Lump Sum	0.0	0.0
Employee Transfers	(12.0)	(8.2)
Increase due to acquisition of Aquarion	0.0	0.0
Fair Value of Pension Plan Assets as of End of Year	918.4	963.0	905.5
Funded Status as of December 31st	(242.0)	(312.2)
The Connecticut Light And Power Company | Other Postretirement Benefits Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	(178.4)	(165.0)
Service Cost	(1.9)	(1.9)	(2.0)
Interest Cost	(5.8)	(5.3)	(5.3)
Actuarial Gain/(Loss)	14.4	(18.5)
Benefits Paid - Pension	10.1	9.9
Employee Transfers	(0.1)	2.4
Increase due to acquisition of Aquarion	0.0	0.0
Benefit Obligation as of End of Year	(161.7)	(178.4)	(165.0)
Change in Pension Plan Assets:
Fair Value of Pension Plan Assets as of Beginning of Year	135.9	129.2
Employer Contributions	0.0	0.0
Actual Return on Pension Plan Assets	(5.2)	18.1
Benefits Paid - Pension	(10.1)	(9.9)
Employee Transfers	0.0	1.5
Increase due to acquisition of Aquarion	0.0	0.0
Fair Value of Pension Plan Assets as of End of Year	120.6	135.9	129.2
Funded Status as of December 31st	(41.1)	(42.5)
NSTAR Electric Company | Pension Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	(1,351.0)	(1,217.3)
Service Cost	(17.4)	(15.5)	(16.3)
Interest Cost	(43.5)	(42.7)	(42.2)
Actuarial Gain/(Loss)	98.6	(143.5)
Benefits Paid - Pension	66.9	55.4
Benefits Paid - Lump Sum	7.1	6.8
Benefits Paid - SERP	0.3	0.3
Employee Transfers	2.5	5.5
Increase due to acquisition of Aquarion	0.0	0.0
Benefit Obligation as of End of Year	(1,236.5)	(1,351.0)	(1,217.3)
Change in Pension Plan Assets:
Fair Value of Pension Plan Assets as of Beginning of Year	1,260.8	1,088.3
Employer Contributions	56.5	85.4
Actual Return on Pension Plan Assets	(18.7)	154.8
Benefits Paid - Pension	(66.9)	(55.4)
Benefits Paid - Lump Sum	(7.1)	(6.8)
Employee Transfers	(2.5)	(5.5)
Increase due to acquisition of Aquarion	0.0	0.0
Fair Value of Pension Plan Assets as of End of Year	1,222.1	1,260.8	1,088.3
Funded Status as of December 31st	(14.4)	(90.2)
NSTAR Electric Company | Other Postretirement Benefits Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	(278.6)	(270.0)
Service Cost	(2.0)	(1.7)	(3.4)
Interest Cost	(8.7)	(8.7)	(13.3)
Actuarial Gain/(Loss)	28.4	(13.2)
Benefits Paid - Pension	14.5	13.5
Employee Transfers	0.1	1.5
Increase due to acquisition of Aquarion	0.0	0.0
Benefit Obligation as of End of Year	(246.3)	(278.6)	(270.0)
Change in Pension Plan Assets:
Fair Value of Pension Plan Assets as of Beginning of Year	405.5	361.6
Employer Contributions	5.2	5.3
Actual Return on Pension Plan Assets	(17.4)	52.9
Benefits Paid - Pension	(14.5)	(13.5)
Employee Transfers	(0.3)	0.8
Increase due to acquisition of Aquarion	0.0	0.0
Fair Value of Pension Plan Assets as of End of Year	379.1	405.5	361.6
Funded Status as of December 31st	132.8	126.9
Public Service Company Of New Hampshire | Pension Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	(642.2)	(572.2)
Service Cost	(11.2)	(9.7)	(9.9)
Interest Cost	(22.0)	(21.2)	(20.7)
Actuarial Gain/(Loss)	39.2	(65.1)
Benefits Paid - Pension	26.2	26.4
Benefits Paid - Lump Sum	0.0	0.0
Benefits Paid - SERP	0.2	0.3
Employee Transfers	(0.9)	(0.7)
Increase due to acquisition of Aquarion	0.0	0.0
Benefit Obligation as of End of Year	(610.7)	(642.2)	(572.2)
Change in Pension Plan Assets:
Fair Value of Pension Plan Assets as of Beginning of Year	539.5	494.0
Employer Contributions	0.0	0.8
Actual Return on Pension Plan Assets	(7.6)	70.4
Benefits Paid - Pension	(26.2)	(26.4)
Benefits Paid - Lump Sum	0.0	0.0
Employee Transfers	0.9	0.7
Increase due to acquisition of Aquarion	0.0	0.0
Fair Value of Pension Plan Assets as of End of Year	506.6	539.5	494.0
Funded Status as of December 31st	(104.1)	(102.7)
Public Service Company Of New Hampshire | Other Postretirement Benefits Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	(101.1)	(89.7)
Service Cost	(1.1)	(1.3)	(1.3)
Interest Cost	(3.4)	(3.0)	(2.9)
Actuarial Gain/(Loss)	8.6	(11.9)
Benefits Paid - Pension	4.9	4.6
Employee Transfers	0.2	0.2
Increase due to acquisition of Aquarion	0.0	0.0
Benefit Obligation as of End of Year	(91.9)	(101.1)	(89.7)
Change in Pension Plan Assets:
Fair Value of Pension Plan Assets as of Beginning of Year	79.0	73.2
Employer Contributions	0.0	0.0
Actual Return on Pension Plan Assets	(2.9)	10.4
Benefits Paid - Pension	(4.9)	(4.6)
Employee Transfers	0.0	0.0
Increase due to acquisition of Aquarion	0.0	0.0
Fair Value of Pension Plan Assets as of End of Year	71.2	79.0	$ 73.2
Funded Status as of December 31st	$ (20.7)	$ (22.1)