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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 | 6 Months Ended
--- | --- | ---
Jun. 30, 2025 | Jun. 30, 2024 | Jun. 30, 2025 | Jun. 30, 2024
Pension and SERP |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Service Cost | $ 17.2 | $ 11.2 | $ 33.6 | $ 22.0
Interest Cost | 64.1 | 62.7 | 127.3 | 124.7
Expected Return on Plan Assets | (113.7) | (115.7) | (227.0) | (231.3)
Actuarial Loss/(Gain) | 11.0 | 22.1 | 21.0 | 41.9
Prior Service Cost/(Credit) | 0.3 | 0.3 | 0.6 | 0.6
Settlement Loss |  |  | 3.3 | 4.3
Total Net Periodic Benefit Plan Income | (21.1) | (19.4) | (41.2) | (37.8)
PBOP |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Service Cost | 1.6 | 1.8 | 3.2 | 3.5
Interest Cost | 8.0 | 8.0 | 16.0 | 16.0
Expected Return on Plan Assets | (21.1) | (20.3) | (42.3) | (40.7)
Actuarial Loss/(Gain) | (0.2) | (0.1) | (0.5) | (0.2)
Prior Service Cost/(Credit) | (5.4) | (5.4) | (10.8) | (10.8)
Settlement Loss |  |  | 0.0 | 0.0
Total Net Periodic Benefit Plan Income | (17.1) | (16.0) | (34.4) | (32.2)
CL&P | Pension and SERP |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Service Cost | 3.8 | 3.3 | 7.6 | 6.2
Interest Cost | 12.9 | 12.7 | 25.7 | 25.1
Expected Return on Plan Assets | (23.0) | (23.5) | (45.9) | (46.8)
Actuarial Loss/(Gain) | 1.3 | 3.2 | 2.5 | 5.7
Prior Service Cost/(Credit) | 0.0 | 0.0 | 0.0 | 0.0
Settlement Loss |  |  | 0.0 | 0.0
Total Net Periodic Benefit Plan Income | (5.0) | (4.3) | (10.1) | (9.8)
Intercompany (Income)/Expense Allocations | (0.5) | (0.6) | (0.4) | (0.3)
CL&P | PBOP |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Service Cost | 0.3 | 0.3 | 0.5 | 0.6
Interest Cost | 1.4 | 1.4 | 2.8 | 2.8
Expected Return on Plan Assets | (2.4) | (2.3) | (4.8) | (4.7)
Actuarial Loss/(Gain) | 0.0 | 0.0 | 0.0 | 0.0
Prior Service Cost/(Credit) | 0.3 | 0.2 | 0.6 | 0.5
Settlement Loss |  |  | 0.0 | 0.0
Total Net Periodic Benefit Plan Income | (0.4) | (0.4) | (0.9) | (0.8)
Intercompany (Income)/Expense Allocations | (0.5) | (0.5) | (1.0) | (1.1)
NSTAR Electric | Pension and SERP |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Service Cost | 2.4 | 1.9 | 4.9 | 4.0
Interest Cost | 13.4 | 12.8 | 26.4 | 25.9
Expected Return on Plan Assets | (27.7) | (28.0) | (55.4) | (56.3)
Actuarial Loss/(Gain) | 3.7 | 6.4 | 6.9 | 12.8
Prior Service Cost/(Credit) | 0.1 | 0.1 | 0.2 | 0.2
Settlement Loss |  |  | 0.0 | 0.0
Total Net Periodic Benefit Plan Income | (8.1) | (6.8) | (17.0) | (13.4)
Intercompany (Income)/Expense Allocations | (0.3) | (0.6) | 0.0 | (0.2)
NSTAR Electric | PBOP |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Service Cost | 0.3 | 0.3 | 0.5 | 0.6
Interest Cost | 2.2 | 2.2 | 4.4 | 4.4
Expected Return on Plan Assets | (10.5) | (9.9) | (21.0) | (19.8)
Actuarial Loss/(Gain) | 0.0 | 0.0 | 0.0 | 0.0
Prior Service Cost/(Credit) | (4.3) | (4.3) | (8.5) | (8.5)
Settlement Loss |  |  | 0.0 | 0.0
Total Net Periodic Benefit Plan Income | (12.3) | (11.7) | (24.6) | (23.3)
Intercompany (Income)/Expense Allocations | (0.7) | (0.6) | (1.4) | (1.3)
PSNH | Pension and SERP |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Service Cost | 1.6 | 1.1 | 3.2 | 2.1
Interest Cost | 6.8 | 6.7 | 13.6 | 13.4
Expected Return on Plan Assets | (12.0) | (12.2) | (23.9) | (24.4)
Actuarial Loss/(Gain) | 0.5 | 1.3 | 1.0 | 2.4
Prior Service Cost/(Credit) | 0.0 | 0.0 | 0.0 | 0.0
Settlement Loss |  |  | 0.0 | 0.0
Total Net Periodic Benefit Plan Income | (3.1) | (3.1) | (6.1) | (6.5)
Intercompany (Income)/Expense Allocations | (0.1) | (0.2) | 0.0 | (0.1)
PSNH | PBOP |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Service Cost | 0.1 | 0.2 | 0.2 | 0.3
Interest Cost | 0.8 | 0.8 | 1.6 | 1.7
Expected Return on Plan Assets | (1.4) | (1.4) | (2.8) | (2.8)
Actuarial Loss/(Gain) | 0.0 | 0.0 | 0.0 | 0.0
Prior Service Cost/(Credit) | 0.1 | 0.1 | 0.2 | 0.2
Settlement Loss |  |  | 0.0 | 0.0
Total Net Periodic Benefit Plan Income | (0.4) | (0.3) | (0.8) | (0.6)
Intercompany (Income)/Expense Allocations | $ (0.2) | $ (0.2) | $ (0.4) | $ (0.4)

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