EMPLOYEE BENEFITS - Schedule of Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Pension and SERP
Defined Benefit Plan Disclosure [Line Items]
Service Cost	$ 43.1	$ 70.1	$ 85.8
Interest Cost	254.0	154.5	130.0
Expected Return on Plan Assets	(465.0)	(523.6)	(437.5)
Actuarial Loss	45.8	116.0	243.9
Prior Service Cost/(Credit)	1.3	1.4	1.4
Settlement Loss	12.4
Total Net Periodic Benefit Plan Income	(108.4)	$ (181.6)	$ 23.6
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Excluding Service Cost, Statement of Income or Comprehensive Income [Extensible Enumeration]		Other Income, Net	Other Income, Net
PBOP
Defined Benefit Plan Disclosure [Line Items]
Service Cost	7.6	$ 11.6	$ 13.5
Interest Cost	33.8	20.2	17.4
Expected Return on Plan Assets	(77.1)	(89.9)	(79.1)
Actuarial Loss	0.0	0.0	8.9
Prior Service Cost/(Credit)	(21.6)	(21.7)	(21.2)
Settlement Loss	0.0
Total Net Periodic Benefit Plan Income	$ (57.3)	$ (79.8)	$ (60.5)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Excluding Service Cost, Statement of Income or Comprehensive Income [Extensible Enumeration]	Other Income, Net	Other Income, Net	Other Income, Net
CL&P | Pension and SERP
Defined Benefit Plan Disclosure [Line Items]
Service Cost	$ 12.3	$ 18.7	$ 23.0
Interest Cost	50.5	31.3	27.3
Expected Return on Plan Assets	(94.2)	(106.3)	(86.8)
Actuarial Loss	2.5	16.2	45.5
Prior Service Cost/(Credit)	0.0	0.0	0.0
Settlement Loss	0.0
Total Net Periodic Benefit Plan Income	(28.9)	(40.1)	9.0
Intercompany Income Allocations	(4.0)	(16.0)	8.0
CL&P | PBOP
Defined Benefit Plan Disclosure [Line Items]
Service Cost	1.3	2.0	2.3
Interest Cost	6.2	3.7	3.2
Expected Return on Plan Assets	(9.4)	(11.4)	(10.3)
Actuarial Loss	0.0	0.0	1.8
Prior Service Cost/(Credit)	1.1	1.1	1.1
Settlement Loss	0.0
Total Net Periodic Benefit Plan Income	(0.8)	(4.6)	(1.9)
Intercompany Income Allocations	(1.9)	(3.7)	(1.6)
NSTAR Electric | Pension and SERP
Defined Benefit Plan Disclosure [Line Items]
Service Cost	7.8	13.8	15.8
Interest Cost	53.9	32.8	26.8
Expected Return on Plan Assets	(113.8)	(128.4)	(108.1)
Actuarial Loss	17.1	32.8	61.6
Prior Service Cost/(Credit)	0.3	0.3	0.3
Settlement Loss	0.0
Total Net Periodic Benefit Plan Income	(34.7)	(48.7)	(3.6)
Intercompany Income Allocations	(3.0)	(12.4)	8.8
NSTAR Electric | PBOP
Defined Benefit Plan Disclosure [Line Items]
Service Cost	1.2	2.0	2.4
Interest Cost	9.2	5.3	4.4
Expected Return on Plan Assets	(36.9)	(42.4)	(36.9)
Actuarial Loss	0.0	0.0	2.4
Prior Service Cost/(Credit)	(17.0)	(17.0)	(17.0)
Settlement Loss	0.0
Total Net Periodic Benefit Plan Income	(43.5)	(52.1)	(44.7)
Intercompany Income Allocations	(2.1)	(3.6)	(1.9)
PSNH | Pension and SERP
Defined Benefit Plan Disclosure [Line Items]
Service Cost	4.3	6.9	8.9
Interest Cost	27.3	16.9	14.5
Expected Return on Plan Assets	(49.5)	(56.1)	(47.5)
Actuarial Loss	1.5	7.9	20.7
Prior Service Cost/(Credit)	0.0	0.0	0.0
Settlement Loss	0.0
Total Net Periodic Benefit Plan Income	(16.4)	(24.4)	(3.4)
Intercompany Income Allocations	(0.8)	(3.6)	2.7
PSNH | PBOP
Defined Benefit Plan Disclosure [Line Items]
Service Cost	0.7	1.1	1.2
Interest Cost	3.7	2.2	1.8
Expected Return on Plan Assets	(5.5)	(6.7)	(6.1)
Actuarial Loss	0.0	0.0	0.7
Prior Service Cost/(Credit)	0.4	0.4	0.4
Settlement Loss	0.0
Total Net Periodic Benefit Plan Income	(0.7)	(3.0)	(2.0)
Intercompany Income Allocations	$ (0.7)	$ (1.2)	$ (0.6)