EMPLOYEE BENEFITS - Schedule of Pension Benefit Obligations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	$ (5,520.0)	$ (5,936.5)
Service Cost	(67.7)	(84.8)	$ (71.3)
Interest Cost	(219.0)	(196.4)	(188.0)
Actuarial Gain/(Loss)	(815.3)	414.9
Benefits Paid	273.0	261.8
Benefits Paid - Lump Sum	20.0	14.2
Benefits Paid - SERP	7.3	6.8
Employee Transfers	0.0	0.0
Benefit Obligation as of End of Year	(6,321.7)	(5,520.0)	(5,936.5)
Change in Pension Plan Assets:
Fair Value of Pension Plan Assets as of Beginning of Year	4,573.9	4,739.5
Employer Contributions	112.5	185.6
Actual Return on Pension Plan Assets	575.2	(75.2)
Benefits Paid - Pension	(273.0)	(261.8)
Benefits Paid - Lump Sum	(20.0)	(14.2)
Employee Transfers	0.0	0.0
Fair Value of Pension Plan Assets as of End of Year	4,968.6	4,573.9	4,739.5
Funded Status as of December 31st	(1,353.1)	(946.1)
Other Postretirement Benefits Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	(841.5)	(948.6)
Service Cost	(7.8)	(10.0)	(9.5)
Interest Cost	(32.7)	(30.7)	(27.1)
Actuarial Gain/(Loss)	(67.0)	102.5
Benefits Paid	50.0	45.3
Employee Transfers	0.0	0.0
Benefit Obligation as of End of Year	(899.0)	(841.5)	(948.6)
Change in Pension Plan Assets:
Fair Value of Pension Plan Assets as of Beginning of Year	849.6	922.2
Employer Contributions	9.3	9.3
Actual Return on Pension Plan Assets	127.0	(36.6)
Benefits Paid - Pension	(50.0)	(45.3)
Employee Transfers	0.0	0.0
Fair Value of Pension Plan Assets as of End of Year	935.9	849.6	922.2
Funded Status as of December 31st	36.9	8.1
CL&P | Pension Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	(1,160.4)	(1,275.2)
Service Cost	(18.0)	(21.4)	(18.5)
Interest Cost	(45.7)	(41.8)	(41.6)
Actuarial Gain/(Loss)	(176.6)	106.1
Benefits Paid	60.2	59.6
Benefits Paid - Lump Sum	0.0	0.0
Benefits Paid - SERP	0.3	0.3
Employee Transfers	8.9	12.0
Benefit Obligation as of End of Year	(1,331.3)	(1,160.4)	(1,275.2)
Change in Pension Plan Assets:
Fair Value of Pension Plan Assets as of Beginning of Year	918.4	963.0
Employer Contributions	24.0	41.2
Actual Return on Pension Plan Assets	112.9	(14.2)
Benefits Paid - Pension	(60.2)	(59.6)
Benefits Paid - Lump Sum	0.0	0.0
Employee Transfers	(8.9)	(12.0)
Fair Value of Pension Plan Assets as of End of Year	986.2	918.4	963.0
Funded Status as of December 31st	(345.1)	(242.0)
CL&P | Other Postretirement Benefits Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	(161.7)	(178.4)
Service Cost	(1.4)	(1.9)	(1.9)
Interest Cost	(6.3)	(5.8)	(5.3)
Actuarial Gain/(Loss)	(13.4)	14.4
Benefits Paid	10.8	10.1
Employee Transfers	(0.7)	(0.1)
Benefit Obligation as of End of Year	(172.7)	(161.7)	(178.4)
Change in Pension Plan Assets:
Fair Value of Pension Plan Assets as of Beginning of Year	120.6	135.9
Employer Contributions	0.0	0.0
Actual Return on Pension Plan Assets	17.1	(5.2)
Benefits Paid - Pension	(10.8)	(10.1)
Employee Transfers	(0.6)	0.0
Fair Value of Pension Plan Assets as of End of Year	126.3	120.6	135.9
Funded Status as of December 31st	(46.4)	(41.1)
NSTAR Electric | Pension Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	(1,236.5)	(1,351.0)
Service Cost	(14.6)	(17.4)	(15.5)
Interest Cost	(49.0)	(43.5)	(42.7)
Actuarial Gain/(Loss)	(181.0)	98.6
Benefits Paid	67.1	66.9
Benefits Paid - Lump Sum	12.9	7.1
Benefits Paid - SERP	0.1	0.3
Employee Transfers	3.7	2.5
Benefit Obligation as of End of Year	(1,397.3)	(1,236.5)	(1,351.0)
Change in Pension Plan Assets:
Fair Value of Pension Plan Assets as of Beginning of Year	1,222.1	1,260.8
Employer Contributions	0.4	56.5
Actual Return on Pension Plan Assets	150.0	(18.7)
Benefits Paid - Pension	(67.1)	(66.9)
Benefits Paid - Lump Sum	(12.9)	(7.1)
Employee Transfers	(3.7)	(2.5)
Fair Value of Pension Plan Assets as of End of Year	1,288.8	1,222.1	1,260.8
Funded Status as of December 31st	(108.5)	(14.4)
NSTAR Electric | Other Postretirement Benefits Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	(246.3)	(278.6)
Service Cost	(1.7)	(2.0)	(1.7)
Interest Cost	(9.5)	(8.7)	(8.7)
Actuarial Gain/(Loss)	(15.2)	28.4
Benefits Paid	15.4	14.5
Employee Transfers	(1.0)	0.1
Benefit Obligation as of End of Year	(258.3)	(246.3)	(278.6)
Change in Pension Plan Assets:
Fair Value of Pension Plan Assets as of Beginning of Year	379.1	405.5
Employer Contributions	6.0	5.2
Actual Return on Pension Plan Assets	57.0	(17.4)
Benefits Paid - Pension	(15.4)	(14.5)
Employee Transfers	(2.3)	0.3
Fair Value of Pension Plan Assets as of End of Year	424.4	379.1	405.5
Funded Status as of December 31st	166.1	132.8
PSNH | Pension Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	(610.7)	(642.2)
Service Cost	(7.1)	(11.2)	(9.7)
Interest Cost	(24.0)	(22.0)	(21.2)
Actuarial Gain/(Loss)	(84.5)	39.2
Benefits Paid	30.3	26.2
Benefits Paid - Lump Sum	0.0	0.0
Benefits Paid - SERP	0.4	0.2
Employee Transfers	3.0	(0.9)
Benefit Obligation as of End of Year	(692.6)	(610.7)	(642.2)
Change in Pension Plan Assets:
Fair Value of Pension Plan Assets as of Beginning of Year	506.6	539.5
Employer Contributions	15.4	0.0
Actual Return on Pension Plan Assets	62.9	(7.6)
Benefits Paid - Pension	(30.3)	(26.2)
Benefits Paid - Lump Sum	0.0	0.0
Employee Transfers	(3.0)	0.9
Fair Value of Pension Plan Assets as of End of Year	551.6	506.6	539.5
Funded Status as of December 31st	(141.0)	(104.1)
PSNH | Other Postretirement Benefits Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	(91.9)	(101.1)
Service Cost	(0.7)	(1.1)	(1.3)
Interest Cost	(3.4)	(3.4)	(3.0)
Actuarial Gain/(Loss)	(3.1)	8.6
Benefits Paid	5.6	4.9
Employee Transfers	0.5	0.2
Benefit Obligation as of End of Year	(93.0)	(91.9)	(101.1)
Change in Pension Plan Assets:
Fair Value of Pension Plan Assets as of Beginning of Year	71.2	79.0
Employer Contributions	0.0	0.0
Actual Return on Pension Plan Assets	10.0	(2.9)
Benefits Paid - Pension	(5.6)	(4.9)
Employee Transfers	0.4	0.0
Fair Value of Pension Plan Assets as of End of Year	76.0	71.2	$ 79.0
Funded Status as of December 31st	$ (17.0)	$ (20.7)