EMPLOYEE BENEFITS - Schedule of Pension Benefit Obligations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	$ (5,242.3)	$ (5,080.1)
Plan Amendment	0.0	(9.0)
Employee Transfers	0.0	0.0
Service Cost	(71.3)	(75.0)	$ (91.4)
Interest Cost	(188.0)	(185.5)	(227.0)
Actuarial Gain/(Loss)	(548.7)	(151.8)
Benefits Paid - Pension	243.7	254.0
Benefits Paid - Lump Sum	(18.4)	0.0
Benefits Paid - SERP	20.4	5.1
Increase due to acquisition of Aquarion	(168.7)	0.0
Benefit Obligation as of End of Year	(5,936.5)	(5,242.3)	(5,080.1)
Change in Pension Plan Assets:
Fair Value of Pension Plan Assets as of Beginning of Year	4,076.0	3,905.4
Increase due to Aquarion Acquisition	100.7	0.0
Employee Transfers	0.0	0.0
Employer Contributions	235.2	146.2
Actual Return on Pension Plan Assets	589.7	278.4
Benefits Paid	(243.7)	(254.0)
Benefits Paid - Lump Sum	(18.4)	0.0
Fair Value of Pension Plan Assets as of End of Year	4,739.5	4,076.0	3,905.4
Funded Status as of December 31st	(1,197.0)	(1,166.3)
Other Postretirement Benefits Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	(810.0)	(1,051.4)
Plan Amendment	0.0	244.0
Employee Transfers	0.0	0.0
Service Cost	(9.5)	(12.2)	(16.3)
Interest Cost	(27.1)	(32.9)	(47.2)
Actuarial Gain/(Loss)	(81.8)	(17.7)
Benefits Paid - Pension	41.5	60.2
Increase due to acquisition of Aquarion	(61.7)	0.0
Benefit Obligation as of End of Year	(948.6)	(810.0)	(1,051.4)
Change in Pension Plan Assets:
Fair Value of Pension Plan Assets as of Beginning of Year	815.8	812.2
Increase due to Aquarion Acquisition	22.3	0.0
Employee Transfers	0.0	0.0
Employer Contributions	7.6	12.5
Actual Return on Pension Plan Assets	118.0	51.3
Benefits Paid	(41.5)	(60.2)
Fair Value of Pension Plan Assets as of End of Year	922.2	815.8	812.2
Funded Status as of December 31st	(26.4)	5.8
The Connecticut Light And Power Company | Pension Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	(1,170.2)	(1,157.6)
Plan Amendment	0.0	0.0
Employee Transfers	8.2	8.8
Service Cost	(18.5)	(18.8)	(24.7)
Interest Cost	(41.6)	(41.6)	(51.1)
Actuarial Gain/(Loss)	(116.9)	(23.9)
Benefits Paid - Pension	63.5	62.6
Benefits Paid - Lump Sum	0.0	0.0
Benefits Paid - SERP	0.3	0.3
Increase due to acquisition of Aquarion	0.0	0.0
Benefit Obligation as of End of Year	(1,275.2)	(1,170.2)	(1,157.6)
Change in Pension Plan Assets:
Fair Value of Pension Plan Assets as of Beginning of Year	905.5	913.5
Increase due to Aquarion Acquisition	0.0	0.0
Employee Transfers	(8.2)	(8.8)
Employer Contributions	2.5	0.4
Actual Return on Pension Plan Assets	126.7	63.0
Benefits Paid	(63.5)	(62.6)
Benefits Paid - Lump Sum	0.0	0.0
Fair Value of Pension Plan Assets as of End of Year	963.0	905.5	913.5
Funded Status as of December 31st	(312.2)	(264.7)
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	(165.0)	(164.0)
Plan Amendment	0.0	(12.5)
Employee Transfers	2.4	1.3
Service Cost	(1.9)	(2.0)	(2.1)
Interest Cost	(5.3)	(5.3)	(7.2)
Actuarial Gain/(Loss)	(18.5)	3.6
Benefits Paid - Pension	9.9	13.9
Increase due to acquisition of Aquarion	0.0	0.0
Benefit Obligation as of End of Year	(178.4)	(165.0)	(164.0)
Change in Pension Plan Assets:
Fair Value of Pension Plan Assets as of Beginning of Year	129.2	136.7
Increase due to Aquarion Acquisition	0.0	0.0
Employee Transfers	(1.5)	(0.8)
Employer Contributions	0.0	0.0
Actual Return on Pension Plan Assets	18.1	7.2
Benefits Paid	(9.9)	(13.9)
Fair Value of Pension Plan Assets as of End of Year	135.9	129.2	136.7
Funded Status as of December 31st	(42.5)	(35.8)
NSTAR Electric Company | Pension Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	(1,217.3)	(1,187.3)
Plan Amendment	0.0	(2.8)
Employee Transfers	5.5	1.3
Service Cost	(15.5)	(16.3)	(19.2)
Interest Cost	(42.7)	(42.2)	(50.6)
Actuarial Gain/(Loss)	(143.5)	(37.2)
Benefits Paid - Pension	55.4	67.0
Benefits Paid - Lump Sum	(6.8)	0.0
Benefits Paid - SERP	0.3	0.2
Increase due to acquisition of Aquarion	0.0	0.0
Benefit Obligation as of End of Year	(1,351.0)	(1,217.3)	(1,187.3)
Change in Pension Plan Assets:
Fair Value of Pension Plan Assets as of Beginning of Year	1,088.3	1,053.7
Increase due to Aquarion Acquisition	0.0	0.0
Employee Transfers	(5.5)	(1.3)
Employer Contributions	85.4	28.4
Actual Return on Pension Plan Assets	154.8	74.5
Benefits Paid	(55.4)	(67.0)
Benefits Paid - Lump Sum	(6.8)	0.0
Fair Value of Pension Plan Assets as of End of Year	1,260.8	1,088.3	1,053.7
Funded Status as of December 31st	(90.2)	(129.0)
NSTAR Electric Company | Other Postretirement Benefits Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	(270.0)	(447.2)
Plan Amendment	0.0	193.6
Employee Transfers	1.5	0.5
Service Cost	(1.7)	(3.4)	(5.8)
Interest Cost	(8.7)	(13.3)	(20.5)
Actuarial Gain/(Loss)	(13.2)	(23.5)
Benefits Paid - Pension	13.5	23.3
Increase due to acquisition of Aquarion	0.0	0.0
Benefit Obligation as of End of Year	(278.6)	(270.0)	(447.2)
Change in Pension Plan Assets:
Fair Value of Pension Plan Assets as of Beginning of Year	361.6	352.0
Increase due to Aquarion Acquisition	0.0	0.0
Employee Transfers	(0.8)	(0.6)
Employer Contributions	5.3	8.9
Actual Return on Pension Plan Assets	52.9	24.6
Benefits Paid	(13.5)	(23.3)
Fair Value of Pension Plan Assets as of End of Year	405.5	361.6	352.0
Funded Status as of December 31st	126.9	91.6
Public Service Company Of New Hampshire | Pension Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	(572.2)	(547.6)
Plan Amendment	0.0	0.0
Employee Transfers	(0.7)	2.4
Service Cost	(9.7)	(9.9)	(12.1)
Interest Cost	(21.2)	(20.7)	(24.3)
Actuarial Gain/(Loss)	(65.1)	(21.5)
Benefits Paid - Pension	26.4	24.9
Benefits Paid - Lump Sum	0.0	0.0
Benefits Paid - SERP	0.3	0.2
Increase due to acquisition of Aquarion	0.0	0.0
Benefit Obligation as of End of Year	(642.2)	(572.2)	(547.6)
Change in Pension Plan Assets:
Fair Value of Pension Plan Assets as of Beginning of Year	494.0	470.5
Increase due to Aquarion Acquisition	0.0	0.0
Employee Transfers	0.7	(2.4)
Employer Contributions	0.8	17.1
Actual Return on Pension Plan Assets	70.4	33.7
Benefits Paid	(26.4)	(24.9)
Benefits Paid - Lump Sum	0.0	0.0
Fair Value of Pension Plan Assets as of End of Year	539.5	494.0	470.5
Funded Status as of December 31st	(102.7)	(78.2)
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	(89.7)	(88.5)
Plan Amendment	0.0	(6.7)
Employee Transfers	0.2	0.3
Service Cost	(1.3)	(1.3)	(1.4)
Interest Cost	(3.0)	(2.9)	(3.9)
Actuarial Gain/(Loss)	(11.9)	3.6
Benefits Paid - Pension	4.6	5.8
Increase due to acquisition of Aquarion	0.0	0.0
Benefit Obligation as of End of Year	(101.1)	(89.7)	(88.5)
Change in Pension Plan Assets:
Fair Value of Pension Plan Assets as of Beginning of Year	73.2	75.8
Increase due to Aquarion Acquisition	0.0	0.0
Employee Transfers	0.0	(0.2)
Employer Contributions	0.0	0.0
Actual Return on Pension Plan Assets	10.4	3.4
Benefits Paid	(4.6)	(5.8)
Fair Value of Pension Plan Assets as of End of Year	79.0	73.2	$ 75.8
Funded Status as of December 31st	$ (22.1)	$ (16.5)