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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, 2023 | Sep. 30, 2022 | Sep. 30, 2023 | Sep. 30, 2022
Pension and SERP |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Service Cost | $ 10.7 | $ 17.5 | $ 32.4 | $ 52.7
Interest Cost | 63.4 | 38.6 | 190.6 | 115.8
Expected Return on Plan Assets | (116.3) | (130.7) | (348.7) | (393.1)
Actuarial Loss | 10.9 | 28.4 | 35.0 | 87.5
Prior Service Cost/(Credit) | 0.3 | 0.4 | 0.9 | 1.1
Settlement Loss | 8.7 |  | 12.4 |
Total Net Periodic Benefit Plan Income | (22.3) | (45.8) | (77.4) | (136.0)
PBOP |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Service Cost | 1.9 | 2.9 | 5.7 | 8.7
Interest Cost | 8.5 | 5.0 | 25.4 | 15.1
Expected Return on Plan Assets | (19.3) | (22.5) | (57.9) | (67.5)
Actuarial Loss | 0.0 | 0.0 | 0.0 | 0.0
Prior Service Cost/(Credit) | (5.4) | (5.4) | (16.2) | (16.2)
Settlement Loss | 0.0 |  | 0.0 |
Total Net Periodic Benefit Plan Income | (14.3) | (20.0) | (43.0) | (59.9)
CL&P | Pension and SERP |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Service Cost | 3.1 | 4.7 | 9.2 | 13.9
Interest Cost | 12.6 | 7.8 | 37.9 | 23.4
Expected Return on Plan Assets | (23.5) | (26.5) | (70.6) | (79.6)
Actuarial Loss | 0.5 | 4.0 | 1.9 | 12.2
Prior Service Cost/(Credit) | 0.0 | 0.0 | 0.0 | 0.0
Settlement Loss | 0.0 |  | 0.0 |
Total Net Periodic Benefit Plan Income | (7.3) | (10.0) | (21.6) | (30.1)
Intercompany Expense/(Income) Allocations | 0.8 | (4.0) | (1.8) | (11.9)
CL&P | PBOP |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Service Cost | 0.4 | 0.5 | 1.0 | 1.5
Interest Cost | 1.5 | 0.9 | 4.6 | 2.7
Expected Return on Plan Assets | (2.3) | (2.8) | (7.0) | (8.4)
Actuarial Loss | 0.0 | 0.0 | 0.0 | 0.0
Prior Service Cost/(Credit) | 0.2 | 0.3 | 0.8 | 0.8
Settlement Loss | 0.0 |  | 0.0 |
Total Net Periodic Benefit Plan Income | (0.2) | (1.1) | (0.6) | (3.4)
Intercompany Expense/(Income) Allocations | (0.5) | (0.9) | (1.4) | (2.7)
NSTAR Electric | Pension and SERP |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Service Cost | 1.9 | 3.4 | 5.9 | 10.4
Interest Cost | 13.5 | 8.2 | 40.5 | 24.5
Expected Return on Plan Assets | (28.4) | (32.1) | (85.3) | (96.2)
Actuarial Loss | 4.0 | 8.1 | 12.9 | 24.7
Prior Service Cost/(Credit) | 0.1 | 0.1 | 0.3 | 0.3
Settlement Loss | 0.0 |  | 0.0 |
Total Net Periodic Benefit Plan Income | (8.9) | (12.3) | (25.7) | (36.3)
Intercompany Expense/(Income) Allocations | 0.9 | (3.2) | (1.2) | (9.3)
NSTAR Electric | PBOP |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Service Cost | 0.3 | 0.5 | 0.9 | 1.5
Interest Cost | 2.3 | 1.3 | 6.9 | 3.9
Expected Return on Plan Assets | (9.2) | (10.6) | (27.7) | (31.8)
Actuarial Loss | 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 |
Total Net Periodic Benefit Plan Income | (10.8) | (13.0) | (32.6) | (39.1)
Intercompany Expense/(Income) Allocations | (0.5) | (0.9) | (1.6) | (2.7)
PSNH | Pension and SERP |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Service Cost | 1.0 | 1.7 | 3.2 | 5.2
Interest Cost | 6.8 | 4.2 | 20.4 | 12.6
Expected Return on Plan Assets | (12.3) | (14.0) | (37.1) | (42.1)
Actuarial Loss | 0.4 | 1.9 | 1.2 | 6.0
Prior Service Cost/(Credit) | 0.0 | 0.0 | 0.0 | 0.0
Settlement Loss | 0.0 |  | 0.0 |
Total Net Periodic Benefit Plan Income | (4.1) | (6.2) | (12.3) | (18.3)
Intercompany Expense/(Income) Allocations | 0.3 | (1.0) | (0.3) | (2.7)
PSNH | PBOP |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Service Cost | 0.2 | 0.3 | 0.6 | 0.8
Interest Cost | 0.9 | 0.5 | 2.7 | 1.6
Expected Return on Plan Assets | (1.4) | (1.7) | (4.2) | (5.0)
Actuarial Loss | 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 |
Total Net Periodic Benefit Plan Income | (0.2) | (0.8) | (0.6) | (2.3)
Intercompany Expense/(Income) Allocations | $ (0.2) | $ (0.3) | $ (0.5) | $ (0.9)

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