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PENSION BENEFITS AND POSTRETIREMENT BENEFITS OTHER THAN PENSION - Components of Net Periodic Benefit Expense/(Income) (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2025
Mar. 31, 2024
Pension and SERP    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Service Cost $ 16.4 $ 10.8
Interest Cost 63.2 62.0
Expected Return on Plan Assets (113.3) (115.6)
Actuarial Loss/(Gain) 10.0 19.8
Prior Service Cost/(Credit) 0.3 0.3
Settlement Loss 3.3 4.3
Total Net Periodic Benefit Plan Income (20.1) (18.4)
PBOP    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Service Cost 1.6 1.7
Interest Cost 8.0 8.0
Expected Return on Plan Assets (21.2) (20.4)
Actuarial Loss/(Gain) (0.3) (0.1)
Prior Service Cost/(Credit) (5.4) (5.4)
Settlement Loss 0.0 0.0
Total Net Periodic Benefit Plan Income (17.3) (16.2)
CL&P | Pension and SERP    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Service Cost 3.8 2.9
Interest Cost 12.8 12.4
Expected Return on Plan Assets (22.9) (23.3)
Actuarial Loss/(Gain) 1.2 2.5
Prior Service Cost/(Credit) 0.0 0.0
Settlement Loss 0.0 0.0
Total Net Periodic Benefit Plan Income (5.1) (5.5)
Intercompany Income Allocations 0.1 0.3
CL&P | PBOP    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Service Cost 0.2 0.3
Interest Cost 1.4 1.4
Expected Return on Plan Assets (2.4) (2.4)
Actuarial Loss/(Gain) 0.0 0.0
Prior Service Cost/(Credit) 0.3 0.3
Settlement Loss 0.0 0.0
Total Net Periodic Benefit Plan Income (0.5) (0.4)
Intercompany Income Allocations (0.5) (0.6)
NSTAR Electric | Pension and SERP    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Service Cost 2.5 2.1
Interest Cost 13.0 13.1
Expected Return on Plan Assets (27.7) (28.3)
Actuarial Loss/(Gain) 3.2 6.4
Prior Service Cost/(Credit) 0.1 0.1
Settlement Loss 0.0 0.0
Total Net Periodic Benefit Plan Income (8.9) (6.6)
Intercompany Income Allocations 0.3 0.4
NSTAR Electric | PBOP    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Service Cost 0.2 0.3
Interest Cost 2.2 2.2
Expected Return on Plan Assets (10.5) (9.9)
Actuarial Loss/(Gain) 0.0 0.0
Prior Service Cost/(Credit) (4.2) (4.2)
Settlement Loss 0.0 0.0
Total Net Periodic Benefit Plan Income (12.3) (11.6)
Intercompany Income Allocations (0.7) (0.7)
PSNH | Pension and SERP    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Service Cost 1.6 1.0
Interest Cost 6.8 6.7
Expected Return on Plan Assets (11.9) (12.2)
Actuarial Loss/(Gain) 0.5 1.1
Prior Service Cost/(Credit) 0.0 0.0
Settlement Loss 0.0 0.0
Total Net Periodic Benefit Plan Income (3.0) (3.4)
Intercompany Income Allocations 0.1 0.1
PSNH | PBOP    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Service Cost 0.1 0.1
Interest Cost 0.8 0.9
Expected Return on Plan Assets (1.4) (1.4)
Actuarial Loss/(Gain) 0.0 0.0
Prior Service Cost/(Credit) 0.1 0.1
Settlement Loss 0.0 0.0
Total Net Periodic Benefit Plan Income (0.4) (0.3)
Intercompany Income Allocations $ (0.2) $ (0.2)