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EMPLOYEE BENEFITS - Schedule of Components of Net Periodic Benefit Costs (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Pension and SERP      
Defined Benefit Plan Disclosure [Line Items]      
Service Cost $ 44.3 $ 43.1 $ 70.1
Interest Cost 250.0 254.0 154.5
Expected Return on Plan Assets (462.6) (465.0) (523.6)
Actuarial Loss/(Gain) 85.9 45.8 116.0
Prior Service Cost/(Credit) 1.3 1.3 1.4
Settlement Loss 4.3 12.4  
Total Net Periodic Benefit Plan Income (76.8) $ (108.4) $ (181.6)
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 7.0 $ 7.6 $ 11.6
Interest Cost 31.9 33.8 20.2
Expected Return on Plan Assets (81.2) (77.1) (89.9)
Actuarial Loss/(Gain) (0.4) 0.0 0.0
Prior Service Cost/(Credit) (21.6) (21.6) (21.7)
Settlement Loss 0.0 0.0  
Total Net Periodic Benefit Plan Income $ (64.3) $ (57.3) $ (79.8)
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 $ 12.7 $ 12.3 $ 18.7
Interest Cost 50.3 50.5 31.3
Expected Return on Plan Assets (93.5) (94.2) (106.3)
Actuarial Loss/(Gain) 12.1 2.5 16.2
Prior Service Cost/(Credit) 0.0 0.0 0.0
Settlement Loss 0.0 0.0  
Total Net Periodic Benefit Plan Income (18.4) (28.9) (40.1)
Intercompany Income Allocations (1.6) (4.0) (16.0)
CL&P | PBOP      
Defined Benefit Plan Disclosure [Line Items]      
Service Cost 1.2 1.3 2.0
Interest Cost 5.6 6.2 3.7
Expected Return on Plan Assets (9.5) (9.4) (11.4)
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  
Total Net Periodic Benefit Plan Income (1.6) (0.8) (4.6)
Intercompany Income Allocations (2.2) (1.9) (3.7)
NSTAR Electric | Pension and SERP      
Defined Benefit Plan Disclosure [Line Items]      
Service Cost 7.7 7.8 13.8
Interest Cost 51.6 53.9 32.8
Expected Return on Plan Assets (112.4) (113.8) (128.4)
Actuarial Loss/(Gain) 25.6 17.1 32.8
Prior Service Cost/(Credit) 0.3 0.3 0.3
Settlement Loss 0.0 0.0  
Total Net Periodic Benefit Plan Income (27.2) (34.7) (48.7)
Intercompany Income Allocations (1.3) (3.0) (12.4)
NSTAR Electric | PBOP      
Defined Benefit Plan Disclosure [Line Items]      
Service Cost 1.2 1.2 2.0
Interest Cost 8.8 9.2 5.3
Expected Return on Plan Assets (39.5) (36.9) (42.4)
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  
Total Net Periodic Benefit Plan Income (46.5) (43.5) (52.1)
Intercompany Income Allocations (2.6) (2.1) (3.6)
PSNH | Pension and SERP      
Defined Benefit Plan Disclosure [Line Items]      
Service Cost 4.3 4.3 6.9
Interest Cost 26.8 27.3 16.9
Expected Return on Plan Assets (48.9) (49.5) (56.1)
Actuarial Loss/(Gain) 5.0 1.5 7.9
Prior Service Cost/(Credit) 0.0 0.0 0.0
Settlement Loss 0.0 0.0  
Total Net Periodic Benefit Plan Income (12.8) (16.4) (24.4)
Intercompany Income Allocations (0.4) (0.8) (3.6)
PSNH | PBOP      
Defined Benefit Plan Disclosure [Line Items]      
Service Cost 0.6 0.7 1.1
Interest Cost 3.4 3.7 2.2
Expected Return on Plan Assets (5.6) (5.5) (6.7)
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  
Total Net Periodic Benefit Plan Income (1.2) (0.7) (3.0)
Intercompany Income Allocations $ (0.9) $ (0.7) $ (1.2)