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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, 2023 | Jun. 30, 2022 | Jun. 30, 2023 | Jun. 30, 2022
Pension and SERP |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Service Cost | $ 10.7 | $ 17.5 | $ 21.7 | $ 35.2
Interest Cost | 63.5 | 38.6 | 127.2 | 77.2
Expected Return on Plan Assets | (116.5) | (130.7) | (232.4) | (262.4)
Actuarial Loss | 11.6 | 28.4 | 24.1 | 59.1
Prior Service Cost/(Credit) | 0.3 | 0.3 | 0.6 | 0.7
Settlement Loss | 3.7 |  | 3.7 |
Total Net Periodic Benefit Plan Income | (26.7) | (45.9) | (55.1) | (90.2)
PBOP |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Service Cost | 2.0 | 3.0 | 3.8 | 5.8
Interest Cost | 8.4 | 5.1 | 16.9 | 10.1
Expected Return on Plan Assets | (19.5) | (22.6) | (38.6) | (45.0)
Actuarial Loss | 0.0 | 0.0 | 0.0 | 0.0
Prior Service Cost/(Credit) | (5.4) | (5.4) | (10.8) | (10.8)
Settlement Loss | 0.0 |  | 0.0 |
Total Net Periodic Benefit Plan Income | (14.5) | (19.9) | (28.7) | (39.9)
CL&P | Pension and SERP |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Service Cost | 3.1 | 4.7 | 6.1 | 9.2
Interest Cost | 12.7 | 7.8 | 25.3 | 15.6
Expected Return on Plan Assets | (23.7) | (26.5) | (47.1) | (53.1)
Actuarial Loss | 0.6 | 4.0 | 1.4 | 8.2
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) | (14.3) | (20.1)
Intercompany Income Allocations | (0.6) | (4.1) | (2.6) | (7.9)
CL&P | PBOP |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Service Cost | 0.3 | 0.5 | 0.6 | 1.0
Interest Cost | 1.5 | 0.9 | 3.1 | 1.8
Expected Return on Plan Assets | (2.3) | (2.8) | (4.7) | (5.6)
Actuarial Loss | 0.0 | 0.0 | 0.0 | 0.0
Prior Service Cost/(Credit) | 0.3 | 0.3 | 0.6 | 0.5
Settlement Loss | 0.0 |  | 0.0 |
Total Net Periodic Benefit Plan Income | (0.2) | (1.1) | (0.4) | (2.3)
Intercompany Income Allocations | (0.4) | (0.9) | (0.9) | (1.8)
NSTAR Electric | Pension and SERP |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Service Cost | 1.9 | 3.4 | 4.0 | 7.0
Interest Cost | 13.5 | 8.2 | 27.0 | 16.3
Expected Return on Plan Assets | (28.5) | (32.1) | (56.9) | (64.1)
Actuarial Loss | 4.0 | 8.1 | 8.9 | 16.6
Prior Service Cost/(Credit) | 0.1 | 0.1 | 0.2 | 0.2
Settlement Loss | 0.0 |  | 0.0 |
Total Net Periodic Benefit Plan Income | (9.0) | (12.3) | (16.8) | (24.0)
Intercompany Income Allocations | (0.4) | (3.2) | (2.1) | (6.1)
NSTAR Electric | PBOP |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Service Cost | 0.3 | 0.5 | 0.6 | 1.0
Interest Cost | 2.3 | 1.3 | 4.6 | 2.6
Expected Return on Plan Assets | (9.3) | (10.6) | (18.5) | (21.2)
Actuarial Loss | 0.0 | 0.0 | 0.0 | 0.0
Prior Service Cost/(Credit) | (4.3) | (4.2) | (8.5) | (8.5)
Settlement Loss | 0.0 |  | 0.0 |
Total Net Periodic Benefit Plan Income | (11.0) | (13.0) | (21.8) | (26.1)
Intercompany Income Allocations | (0.5) | (0.9) | (1.1) | (1.8)
PSNH | Pension and SERP |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Service Cost | 1.1 | 1.7 | 2.2 | 3.5
Interest Cost | 6.8 | 4.2 | 13.6 | 8.4
Expected Return on Plan Assets | (12.5) | (14.0) | (24.8) | (28.1)
Actuarial Loss | 0.4 | 1.9 | 0.8 | 4.1
Prior Service Cost/(Credit) | 0.0 | 0.0 | 0.0 | 0.0
Settlement Loss | 0.0 |  | 0.0 |
Total Net Periodic Benefit Plan Income | (4.2) | (6.2) | (8.2) | (12.1)
Intercompany Income Allocations | (0.1) | (0.9) | (0.6) | (1.7)
PSNH | PBOP |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Service Cost | 0.2 | 0.3 | 0.4 | 0.5
Interest Cost | 0.9 | 0.6 | 1.8 | 1.1
Expected Return on Plan Assets | (1.4) | (1.7) | (2.8) | (3.3)
Actuarial Loss | 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 | (0.2) | (0.7) | (0.4) | (1.5)
Intercompany Income Allocations | $ (0.1) | $ (0.3) | $ (0.3) | $ (0.6)

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