EMPLOYEE BENEFITS - Schedule of Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Pension and SERP
Defined Benefit Plan Disclosure [Line Items]
Service Cost	$ 67.9	$ 44.3	$ 43.1
Interest Cost	255.6	250.0	254.0
Expected Return on Plan Assets	(454.3)	(462.6)	(465.0)
Actuarial Loss/(Gain)	42.9	85.9	45.8
Prior Service Cost/(Credit)	1.1	1.3	1.3
Settlement Loss	3.3	4.3	12.4
Total Net Periodic Benefit Plan Income	(83.5)	$ (76.8)	$ (108.4)
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	6.4	$ 7.0	$ 7.6
Interest Cost	32.0	31.9	33.8
Expected Return on Plan Assets	(84.6)	(81.2)	(77.1)
Actuarial Loss/(Gain)	(0.9)	(0.4)	0.0
Prior Service Cost/(Credit)	(21.6)	(21.6)	(21.6)
Settlement Loss	0.0	0.0	0.0
Total Net Periodic Benefit Plan Income	$ (68.7)	$ (64.3)	$ (57.3)
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	$ 15.3	$ 12.7	$ 12.3
Interest Cost	51.5	50.3	50.5
Expected Return on Plan Assets	(91.8)	(93.5)	(94.2)
Actuarial Loss/(Gain)	5.1	12.1	2.5
Prior Service Cost/(Credit)	0.0	0.0	0.0
Settlement Loss	0.0	0.0	0.0
Total Net Periodic Benefit Plan Income	(19.9)	(18.4)	(28.9)
Intercompany Income Allocations	(1.4)	(1.6)	(4.0)
CL&P | PBOP
Defined Benefit Plan Disclosure [Line Items]
Service Cost	1.1	1.2	1.3
Interest Cost	5.6	5.6	6.2
Expected Return on Plan Assets	(9.6)	(9.5)	(9.4)
Actuarial Loss/(Gain)	0.0	0.0	0.0
Prior Service Cost/(Credit)	1.1	1.1	1.1
Settlement Loss	0.0	0.0	0.0
Total Net Periodic Benefit Plan Income	(1.8)	(1.6)	(0.8)
Intercompany Income Allocations	(2.1)	(2.2)	(1.9)
NSTAR Electric | Pension and SERP
Defined Benefit Plan Disclosure [Line Items]
Service Cost	9.7	7.7	7.8
Interest Cost	53.0	51.6	53.9
Expected Return on Plan Assets	(110.7)	(112.4)	(113.8)
Actuarial Loss/(Gain)	14.3	25.6	17.1
Prior Service Cost/(Credit)	0.3	0.3	0.3
Settlement Loss	0.0	0.0	0.0
Total Net Periodic Benefit Plan Income	(33.4)	(27.2)	(34.7)
Intercompany Income Allocations	(0.5)	(1.3)	(3.0)
NSTAR Electric | PBOP
Defined Benefit Plan Disclosure [Line Items]
Service Cost	1.1	1.2	1.2
Interest Cost	8.7	8.8	9.2
Expected Return on Plan Assets	(42.0)	(39.5)	(36.9)
Actuarial Loss/(Gain)	0.0	0.0	0.0
Prior Service Cost/(Credit)	(17.0)	(17.0)	(17.0)
Settlement Loss	0.0	0.0	0.0
Total Net Periodic Benefit Plan Income	(49.2)	(46.5)	(43.5)
Intercompany Income Allocations	(2.7)	(2.6)	(2.1)
PSNH | Pension and SERP
Defined Benefit Plan Disclosure [Line Items]
Service Cost	6.5	4.3	4.3
Interest Cost	27.1	26.8	27.3
Expected Return on Plan Assets	(47.8)	(48.9)	(49.5)
Actuarial Loss/(Gain)	2.1	5.0	1.5
Prior Service Cost/(Credit)	0.0	0.0	0.0
Settlement Loss	0.0	0.0	0.0
Total Net Periodic Benefit Plan Income	(12.1)	(12.8)	(16.4)
Intercompany Income Allocations	(0.1)	(0.4)	(0.8)
PSNH | PBOP
Defined Benefit Plan Disclosure [Line Items]
Service Cost	0.5	0.6	0.7
Interest Cost	3.4	3.4	3.7
Expected Return on Plan Assets	(5.7)	(5.6)	(5.5)
Actuarial Loss/(Gain)	0.0	0.0	0.0
Prior Service Cost/(Credit)	0.4	0.4	0.4
Settlement Loss	0.0	0.0	0.0
Total Net Periodic Benefit Plan Income	(1.4)	(1.2)	(0.7)
Intercompany Income Allocations	$ (0.9)	$ (0.9)	$ (0.7)