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EMPLOYEE BENEFITS - Schedule of Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020
Pension and SERP |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Service Cost | $ 70.1 | $ 85.8 | $ 76.2
Interest Cost | 154.5 | 130.0 | 177.8
Expected Return on Plan Assets | (523.6) | (437.5) | (400.3)
Actuarial Loss | 116.0 | 243.9 | 202.0
Prior Service Cost/(Credit) | 1.4 | 1.4 | 1.2
Total Net Periodic Benefit Plan Expense/(Income) | $ (181.6) | $ 23.6 | 56.9
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 | $ 11.6 | $ 13.5 | 10.2
Interest Cost | 20.2 | 17.4 | 24.6
Expected Return on Plan Assets | (89.9) | (79.1) | (73.6)
Actuarial Loss | 0.0 | 8.9 | 8.4
Prior Service Cost/(Credit) | (21.7) | (21.2) | (21.2)
Total Net Periodic Benefit Plan Expense/(Income) | $ (79.8) | $ (60.5) | (51.6)
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 |
CL&P | Pension and SERP |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Service Cost | $ 18.7 | $ 23.0 | 21.8
Interest Cost | 31.3 | 27.3 | 37.3
Expected Return on Plan Assets | (106.3) | (86.8) | (79.2)
Actuarial Loss | 16.2 | 45.5 | 39.2
Prior Service Cost/(Credit) | 0.0 | 0.0 | 0.0
Total Net Periodic Benefit Plan Expense/(Income) | (40.1) | 9.0 | 19.1
Intercompany Income Allocations | (16.0) | 8.0 | 9.1
CL&P | PBOP |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Service Cost | 2.0 | 2.3 | 1.7
Interest Cost | 3.7 | 3.2 | 4.4
Expected Return on Plan Assets | (11.4) | (10.3) | (9.9)
Actuarial Loss | 0.0 | 1.8 | 1.1
Prior Service Cost/(Credit) | 1.1 | 1.1 | 1.1
Total Net Periodic Benefit Plan Expense/(Income) | (4.6) | (1.9) | (1.6)
Intercompany Income Allocations | (3.7) | (1.6) | (1.1)
NSTAR Electric | Pension and SERP |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Service Cost | 13.8 | 15.8 | 15.4
Interest Cost | 32.8 | 26.8 | 38.6
Expected Return on Plan Assets | (128.4) | (108.1) | (103.0)
Actuarial Loss | 32.8 | 61.6 | 55.2
Prior Service Cost/(Credit) | 0.3 | 0.3 | 0.3
Total Net Periodic Benefit Plan Expense/(Income) | (48.7) | (3.6) | 6.5
Intercompany Income Allocations | (12.4) | 8.8 | 8.9
NSTAR Electric | PBOP |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Service Cost | 2.0 | 2.4 | 2.1
Interest Cost | 5.3 | 4.4 | 6.6
Expected Return on Plan Assets | (42.4) | (36.9) | (34.0)
Actuarial Loss | 0.0 | 2.4 | 2.5
Prior Service Cost/(Credit) | (17.0) | (17.0) | (17.0)
Total Net Periodic Benefit Plan Expense/(Income) | (52.1) | (44.7) | (39.8)
Intercompany Income Allocations | (3.6) | (1.9) | (1.4)
PSNH | Pension and SERP |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Service Cost | 6.9 | 8.9 | 8.2
Interest Cost | 16.9 | 14.5 | 19.4
Expected Return on Plan Assets | (56.1) | (47.5) | (44.7)
Actuarial Loss | 7.9 | 20.7 | 15.6
Prior Service Cost/(Credit) | 0.0 | 0.0 | 0.0
Total Net Periodic Benefit Plan Expense/(Income) | (24.4) | (3.4) | (1.5)
Intercompany Income Allocations | (3.6) | 2.7 | 2.9
PSNH | PBOP |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Service Cost | 1.1 | 1.2 | 0.9
Interest Cost | 2.2 | 1.8 | 2.8
Expected Return on Plan Assets | (6.7) | (6.1) | (5.7)
Actuarial Loss | 0.0 | 0.7 | 0.8
Prior Service Cost/(Credit) | 0.4 | 0.4 | 0.4
Total Net Periodic Benefit Plan Expense/(Income) | (3.0) | (2.0) | (0.8)
Intercompany Income Allocations | $ (1.2) | $ (0.6) | $ (0.5)

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