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EMPLOYEE BENEFITS - Schedule of Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Pension and SERP |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Service Cost | $ 43.1 | $ 70.1 | $ 85.8
Interest Cost | 254.0 | 154.5 | 130.0
Expected Return on Plan Assets | (465.0) | (523.6) | (437.5)
Actuarial Loss | 45.8 | 116.0 | 243.9
Prior Service Cost/(Credit) | 1.3 | 1.4 | 1.4
Settlement Loss | 12.4 |  |
Total Net Periodic Benefit Plan Income | (108.4) | $ (181.6) | $ 23.6
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Excluding Service Cost, Statement of Income or Comprehensive Income [Extensible Enumeration] |  | Other Income, Net | Other Income, Net
PBOP |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Service Cost | 7.6 | $ 11.6 | $ 13.5
Interest Cost | 33.8 | 20.2 | 17.4
Expected Return on Plan Assets | (77.1) | (89.9) | (79.1)
Actuarial Loss | 0.0 | 0.0 | 8.9
Prior Service Cost/(Credit) | (21.6) | (21.7) | (21.2)
Settlement Loss | 0.0 |  |
Total Net Periodic Benefit Plan Income | $ (57.3) | $ (79.8) | $ (60.5)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Excluding Service Cost, Statement of Income or Comprehensive Income [Extensible Enumeration] | Other Income, Net | Other Income, Net | Other Income, Net
CL&P | Pension and SERP |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Service Cost | $ 12.3 | $ 18.7 | $ 23.0
Interest Cost | 50.5 | 31.3 | 27.3
Expected Return on Plan Assets | (94.2) | (106.3) | (86.8)
Actuarial Loss | 2.5 | 16.2 | 45.5
Prior Service Cost/(Credit) | 0.0 | 0.0 | 0.0
Settlement Loss | 0.0 |  |
Total Net Periodic Benefit Plan Income | (28.9) | (40.1) | 9.0
Intercompany Income Allocations | (4.0) | (16.0) | 8.0
CL&P | PBOP |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Service Cost | 1.3 | 2.0 | 2.3
Interest Cost | 6.2 | 3.7 | 3.2
Expected Return on Plan Assets | (9.4) | (11.4) | (10.3)
Actuarial Loss | 0.0 | 0.0 | 1.8
Prior Service Cost/(Credit) | 1.1 | 1.1 | 1.1
Settlement Loss | 0.0 |  |
Total Net Periodic Benefit Plan Income | (0.8) | (4.6) | (1.9)
Intercompany Income Allocations | (1.9) | (3.7) | (1.6)
NSTAR Electric | Pension and SERP |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Service Cost | 7.8 | 13.8 | 15.8
Interest Cost | 53.9 | 32.8 | 26.8
Expected Return on Plan Assets | (113.8) | (128.4) | (108.1)
Actuarial Loss | 17.1 | 32.8 | 61.6
Prior Service Cost/(Credit) | 0.3 | 0.3 | 0.3
Settlement Loss | 0.0 |  |
Total Net Periodic Benefit Plan Income | (34.7) | (48.7) | (3.6)
Intercompany Income Allocations | (3.0) | (12.4) | 8.8
NSTAR Electric | PBOP |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Service Cost | 1.2 | 2.0 | 2.4
Interest Cost | 9.2 | 5.3 | 4.4
Expected Return on Plan Assets | (36.9) | (42.4) | (36.9)
Actuarial Loss | 0.0 | 0.0 | 2.4
Prior Service Cost/(Credit) | (17.0) | (17.0) | (17.0)
Settlement Loss | 0.0 |  |
Total Net Periodic Benefit Plan Income | (43.5) | (52.1) | (44.7)
Intercompany Income Allocations | (2.1) | (3.6) | (1.9)
PSNH | Pension and SERP |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Service Cost | 4.3 | 6.9 | 8.9
Interest Cost | 27.3 | 16.9 | 14.5
Expected Return on Plan Assets | (49.5) | (56.1) | (47.5)
Actuarial Loss | 1.5 | 7.9 | 20.7
Prior Service Cost/(Credit) | 0.0 | 0.0 | 0.0
Settlement Loss | 0.0 |  |
Total Net Periodic Benefit Plan Income | (16.4) | (24.4) | (3.4)
Intercompany Income Allocations | (0.8) | (3.6) | 2.7
PSNH | PBOP |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Service Cost | 0.7 | 1.1 | 1.2
Interest Cost | 3.7 | 2.2 | 1.8
Expected Return on Plan Assets | (5.5) | (6.7) | (6.1)
Actuarial Loss | 0.0 | 0.0 | 0.7
Prior Service Cost/(Credit) | 0.4 | 0.4 | 0.4
Settlement Loss | 0.0 |  |
Total Net Periodic Benefit Plan Income | (0.7) | (3.0) | (2.0)
Intercompany Income Allocations | $ (0.7) | $ (1.2) | $ (0.6)

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