EMPLOYEE BENEFITS - Schedule of Pension Benefit Obligations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Pension and SERP
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	$ (5,220.1)	$ (6,729.7)
Service Cost	(43.1)	(70.1)	$ (85.8)
Interest Cost	(254.0)	(154.5)	(130.0)
Actuarial Gain/(Loss)	(110.4)	1,385.8
Benefits Paid	317.3	302.5
Benefits Paid - Lump Sum	28.9	33.0
Benefits Paid - SERP	43.0	12.9
Employee Transfers	0.0	0.0
Benefit Obligation as of End of Year	(5,238.4)	(5,220.1)	(6,729.7)
Change in Pension Plan Assets:
Fair Value of Plan Assets as of Beginning of Year	5,806.4	6,495.5
Employer Contributions	5.0	80.0
Actual Return on Plan Assets	309.8	(433.6)
Benefits Paid - Pension	(317.3)	(302.5)
Benefits Paid - Lump Sum	(28.9)	(33.0)
Employee Transfers	0.0	0.0
Fair Value of Plan Assets as of End of Year	5,775.0	5,806.4	6,495.5
Funded Status as of December 31st	536.6	586.3
PBOP
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	(693.7)	(884.3)
Service Cost	(7.6)	(11.6)	(13.5)
Interest Cost	(33.8)	(20.2)	(17.4)
Actuarial Gain/(Loss)	5.0	173.6
Benefits Paid	52.7	52.1
Employee Transfers	0.0	0.0
Plan Amendment	1.4	0.0
Impact of Acquisitions	0.0	(3.3)
Benefit Obligation as of End of Year	(676.0)	(693.7)	(884.3)
Change in Pension Plan Assets:
Fair Value of Plan Assets as of Beginning of Year	970.1	1,138.3
Employer Contributions	1.9	3.1
Actual Return on Plan Assets	104.7	(119.6)
Benefits Paid - Pension	(52.3)	(51.7)
Employee Transfers	0.0	0.0
Fair Value of Plan Assets as of End of Year	1,024.4	970.1	1,138.3
Funded Status as of December 31st	348.4	276.4
CL&P | Pension and SERP
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	(1,030.0)	(1,330.9)
Service Cost	(12.3)	(18.7)	(23.0)
Interest Cost	(50.5)	(31.3)	(27.3)
Actuarial Gain/(Loss)	(19.7)	284.1
Benefits Paid	66.1	63.3
Benefits Paid - Lump Sum	0.0	0.0
Benefits Paid - SERP	0.3	0.3
Employee Transfers	(2.4)	3.2
Benefit Obligation as of End of Year	(1,048.5)	(1,030.0)	(1,330.9)
Change in Pension Plan Assets:
Fair Value of Plan Assets as of Beginning of Year	1,172.0	1,323.8
Employer Contributions	0.0	0.0
Actual Return on Plan Assets	61.7	(85.3)
Benefits Paid - Pension	(66.1)	(63.3)
Benefits Paid - Lump Sum	0.0	0.0
Employee Transfers	2.4	(3.2)
Fair Value of Plan Assets as of End of Year	1,170.0	1,172.0	1,323.8
Funded Status as of December 31st	121.5	142.0
CL&P | PBOP
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	(127.9)	(165.5)
Service Cost	(1.3)	(2.0)	(2.3)
Interest Cost	(6.2)	(3.7)	(3.2)
Actuarial Gain/(Loss)	4.4	33.0
Benefits Paid	10.0	10.4
Employee Transfers	0.0	(0.1)
Plan Amendment	0.4	0.0
Impact of Acquisitions	0.0	0.0
Benefit Obligation as of End of Year	(120.6)	(127.9)	(165.5)
Change in Pension Plan Assets:
Fair Value of Plan Assets as of Beginning of Year	120.6	145.7
Employer Contributions	0.0	0.0
Actual Return on Plan Assets	12.6	(15.0)
Benefits Paid - Pension	(10.0)	(10.4)
Employee Transfers	(0.2)	0.3
Fair Value of Plan Assets as of End of Year	123.0	120.6	145.7
Funded Status as of December 31st	2.4	(7.3)
NSTAR Electric | Pension and SERP
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	(1,110.4)	(1,448.4)
Service Cost	(7.8)	(13.8)	(15.8)
Interest Cost	(53.9)	(32.8)	(26.8)
Actuarial Gain/(Loss)	(17.6)	295.5
Benefits Paid	76.7	68.4
Benefits Paid - Lump Sum	5.3	14.8
Benefits Paid - SERP	0.2	0.2
Employee Transfers	0.5	5.7
Benefit Obligation as of End of Year	(1,107.0)	(1,110.4)	(1,448.4)
Change in Pension Plan Assets:
Fair Value of Plan Assets as of Beginning of Year	1,418.8	1,596.0
Employer Contributions	0.0	15.0
Actual Return on Plan Assets	75.3	(103.3)
Benefits Paid - Pension	(76.7)	(68.4)
Benefits Paid - Lump Sum	(5.3)	(14.8)
Employee Transfers	(0.5)	(5.7)
Fair Value of Plan Assets as of End of Year	1,411.6	1,418.8	1,596.0
Funded Status as of December 31st	304.6	308.4
NSTAR Electric | PBOP
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	(189.5)	(238.4)
Service Cost	(1.2)	(2.0)	(2.4)
Interest Cost	(9.2)	(5.3)	(4.4)
Actuarial Gain/(Loss)	(5.8)	39.4
Benefits Paid	16.7	16.6
Employee Transfers	0.5	0.2
Plan Amendment	0.2	0.0
Impact of Acquisitions	0.0	0.0
Benefit Obligation as of End of Year	(188.3)	(189.5)	(238.4)
Change in Pension Plan Assets:
Fair Value of Plan Assets as of Beginning of Year	456.1	530.0
Employer Contributions	0.0	0.0
Actual Return on Plan Assets	52.3	(57.0)
Benefits Paid - Pension	(16.7)	(16.6)
Employee Transfers	(1.3)	(0.3)
Fair Value of Plan Assets as of End of Year	490.4	456.1	530.0
Funded Status as of December 31st	302.1	266.6
PSNH | Pension and SERP
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	(556.2)	(721.0)
Service Cost	(4.3)	(6.9)	(8.9)
Interest Cost	(27.3)	(16.9)	(14.5)
Actuarial Gain/(Loss)	(11.6)	156.7
Benefits Paid	35.7	33.9
Benefits Paid - Lump Sum	1.0	1.0
Benefits Paid - SERP	0.4	0.4
Employee Transfers	0.0	(3.4)
Benefit Obligation as of End of Year	(562.3)	(556.2)	(721.0)
Change in Pension Plan Assets:
Fair Value of Plan Assets as of Beginning of Year	618.0	694.6
Employer Contributions	0.0	0.0
Actual Return on Plan Assets	32.7	(45.1)
Benefits Paid - Pension	(35.7)	(33.9)
Benefits Paid - Lump Sum	(1.0)	(1.0)
Employee Transfers	0.0	3.4
Fair Value of Plan Assets as of End of Year	614.0	618.0	694.6
Funded Status as of December 31st	51.7	61.8
PSNH | PBOP
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	(74.6)	(92.3)
Service Cost	(0.7)	(1.1)	(1.2)
Interest Cost	(3.7)	(2.2)	(1.8)
Actuarial Gain/(Loss)	0.8	15.2
Benefits Paid	6.1	6.0
Employee Transfers	(0.1)	(0.2)
Plan Amendment	0.2	0.0
Impact of Acquisitions	0.0	0.0
Benefit Obligation as of End of Year	(72.0)	(74.6)	(92.3)
Change in Pension Plan Assets:
Fair Value of Plan Assets as of Beginning of Year	72.3	88.0
Employer Contributions	0.0	0.0
Actual Return on Plan Assets	8.3	(9.8)
Benefits Paid - Pension	(6.1)	(6.0)
Employee Transfers	0.2	0.1
Fair Value of Plan Assets as of End of Year	74.7	72.3	$ 88.0
Funded Status as of December 31st	$ 2.7	$ (2.3)