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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 | 9 Months Ended
--- | --- | ---
Sep. 30, 2024 | Sep. 30, 2023 | Sep. 30, 2024 | Sep. 30, 2023
Pension and SERP |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Service Cost | $ 11.2 | $ 10.7 | $ 33.2 | $ 32.4
Interest Cost | 62.7 | 63.4 | 187.4 | 190.6
Expected Return on Plan Assets | (115.7) | (116.3) | (347.0) | (348.7)
Actuarial Loss/(Gain) | 22.0 | 10.9 | 63.9 | 35.0
Prior Service Cost/(Credit) | 0.3 | 0.3 | 0.9 | 0.9
Settlement Loss |  | 8.7 | 4.3 | 12.4
Total Net Periodic Benefit Plan Income | (19.5) | (22.3) | (57.3) | (77.4)
PBOP |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Service Cost | 1.7 | 1.9 | 5.2 | 5.7
Interest Cost | 8.0 | 8.5 | 24.0 | 25.4
Expected Return on Plan Assets | (20.3) | (19.3) | (61.0) | (57.9)
Actuarial Loss/(Gain) | (0.1) | 0.0 | (0.3) | 0.0
Prior Service Cost/(Credit) | (5.4) | (5.4) | (16.2) | (16.2)
Settlement Loss |  | 0.0 | 0.0 | 0.0
Total Net Periodic Benefit Plan Income | (16.1) | (14.3) | (48.3) | (43.0)
CL&P | Pension and SERP |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Service Cost | 3.3 | 3.1 | 9.5 | 9.2
Interest Cost | 12.6 | 12.6 | 37.7 | 37.9
Expected Return on Plan Assets | (23.4) | (23.5) | (70.2) | (70.6)
Actuarial Loss/(Gain) | 3.2 | 0.5 | 8.9 | 1.9
Prior Service Cost/(Credit) | 0.0 | 0.0 | 0.0 | 0.0
Settlement Loss |  | 0.0 | 0.0 | 0.0
Total Net Periodic Benefit Plan Income | (4.3) | (7.3) | (14.1) | (21.6)
Intercompany Income Allocations | (0.6) | 0.8 | (0.9) | (1.8)
CL&P | PBOP |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Service Cost | 0.3 | 0.4 | 0.9 | 1.0
Interest Cost | 1.4 | 1.5 | 4.2 | 4.6
Expected Return on Plan Assets | (2.4) | (2.3) | (7.1) | (7.0)
Actuarial Loss/(Gain) | 0.0 | 0.0 | 0.0 | 0.0
Prior Service Cost/(Credit) | 0.3 | 0.2 | 0.8 | 0.8
Settlement Loss |  | 0.0 | 0.0 | 0.0
Total Net Periodic Benefit Plan Income | (0.4) | (0.2) | (1.2) | (0.6)
Intercompany Income Allocations | (0.6) | (0.5) | (1.7) | (1.4)
NSTAR Electric | Pension and SERP |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Service Cost | 1.9 | 1.9 | 5.9 | 5.9
Interest Cost | 12.8 | 13.5 | 38.7 | 40.5
Expected Return on Plan Assets | (28.0) | (28.4) | (84.3) | (85.3)
Actuarial Loss/(Gain) | 6.4 | 4.0 | 19.2 | 12.9
Prior Service Cost/(Credit) | 0.1 | 0.1 | 0.3 | 0.3
Settlement Loss |  | 0.0 | 0.0 | 0.0
Total Net Periodic Benefit Plan Income | (6.8) | (8.9) | (20.2) | (25.7)
Intercompany Income Allocations | (0.6) | 0.9 | (0.8) | (1.2)
NSTAR Electric | PBOP |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Service Cost | 0.3 | 0.3 | 0.9 | 0.9
Interest Cost | 2.2 | 2.3 | 6.6 | 6.9
Expected Return on Plan Assets | (9.9) | (9.2) | (29.7) | (27.7)
Actuarial Loss/(Gain) | 0.0 | 0.0 | 0.0 | 0.0
Prior Service Cost/(Credit) | (4.2) | (4.2) | (12.7) | (12.7)
Settlement Loss |  | 0.0 | 0.0 | 0.0
Total Net Periodic Benefit Plan Income | (11.6) | (10.8) | (34.9) | (32.6)
Intercompany Income Allocations | (0.6) | (0.5) | (1.9) | (1.6)
PSNH | Pension and SERP |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Service Cost | 1.1 | 1.0 | 3.2 | 3.2
Interest Cost | 6.7 | 6.8 | 20.1 | 20.4
Expected Return on Plan Assets | (12.2) | (12.3) | (36.6) | (37.1)
Actuarial Loss/(Gain) | 1.3 | 0.4 | 3.7 | 1.2
Prior Service Cost/(Credit) | 0.0 | 0.0 | 0.0 | 0.0
Settlement Loss |  | 0.0 | 0.0 | 0.0
Total Net Periodic Benefit Plan Income | (3.1) | (4.1) | (9.6) | (12.3)
Intercompany Income Allocations | (0.2) | 0.3 | (0.3) | (0.3)
PSNH | PBOP |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Service Cost | 0.1 | 0.2 | 0.4 | 0.6
Interest Cost | 0.9 | 0.9 | 2.6 | 2.7
Expected Return on Plan Assets | (1.4) | (1.4) | (4.2) | (4.2)
Actuarial Loss/(Gain) | 0.0 | 0.0 | 0.0 | 0.0
Prior Service Cost/(Credit) | 0.1 | 0.1 | 0.3 | 0.3
Settlement Loss |  | 0.0 | 0.0 | 0.0
Total Net Periodic Benefit Plan Income | (0.3) | (0.2) | (0.9) | (0.6)
Intercompany Income Allocations | $ (0.2) | $ (0.2) | $ (0.6) | $ (0.5)

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