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EMPLOYEE BENEFITS - Schedule of Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019
Pension Plan |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Service Cost | $ 85.8 | $ 76.2 | $ 67.7
Interest Cost | 130.0 | 177.8 | 219.0
Expected Return on Plan Assets | (437.5) | (400.3) | (367.1)
Actuarial Loss | 243.9 | 202.0 | 143.2
Prior Service Cost/(Credit) | 1.4 | 1.2 | 0.9
Total Net Periodic Benefit Plan Expense/(Income) | 23.6 | 56.9 | 63.7
Other Postretirement Benefits Plan |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Service Cost | 13.5 | 10.2 | 7.8
Interest Cost | 17.4 | 24.6 | 32.7
Expected Return on Plan Assets | (79.1) | (73.6) | (66.8)
Actuarial Loss | 8.9 | 8.4 | 8.3
Prior Service Cost/(Credit) | (21.2) | (21.2) | (23.5)
Total Net Periodic Benefit Plan Expense/(Income) | (60.5) | (51.6) | (41.5)
The Connecticut Light and Power Company | Pension Plan |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Service Cost | 23.0 | 21.8 | 18.0
Interest Cost | 27.3 | 37.3 | 45.7
Expected Return on Plan Assets | (86.8) | (79.2) | (73.2)
Actuarial Loss | 45.5 | 39.2 | 26.9
Prior Service Cost/(Credit) | 0.0 | 0.0 | 0.0
Total Net Periodic Benefit Plan Expense/(Income) | 9.0 | 19.1 | 17.4
Intercompany Expense/(Income) Allocations | 8.0 | 9.1 | 8.5
The Connecticut Light and Power Company | Other Postretirement Benefits Plan |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Service Cost | 2.3 | 1.7 | 1.4
Interest Cost | 3.2 | 4.4 | 6.3
Expected Return on Plan Assets | (10.3) | (9.9) | (9.2)
Actuarial Loss | 1.8 | 1.1 | 1.3
Prior Service Cost/(Credit) | 1.1 | 1.1 | 1.1
Total Net Periodic Benefit Plan Expense/(Income) | (1.9) | (1.6) | 0.9
Intercompany Expense/(Income) Allocations | (1.6) | (1.1) | (0.9)
NSTAR Electric Company | Pension Plan |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Service Cost | 15.8 | 15.4 | 14.6
Interest Cost | 26.8 | 38.6 | 49.0
Expected Return on Plan Assets | (108.1) | (103.0) | (97.1)
Actuarial Loss | 61.6 | 55.2 | 44.7
Prior Service Cost/(Credit) | 0.3 | 0.3 | 0.3
Total Net Periodic Benefit Plan Expense/(Income) | (3.6) | 6.5 | 11.5
Intercompany Expense/(Income) Allocations | 8.8 | 8.9 | 8.0
NSTAR Electric Company | Other Postretirement Benefits Plan |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Service Cost | 2.4 | 2.1 | 1.7
Interest Cost | 4.4 | 6.6 | 9.5
Expected Return on Plan Assets | (36.9) | (34.0) | (30.2)
Actuarial Loss | 2.4 | 2.5 | 3.3
Prior Service Cost/(Credit) | (17.0) | (17.0) | (16.9)
Total Net Periodic Benefit Plan Expense/(Income) | (44.7) | (39.8) | (32.6)
Intercompany Expense/(Income) Allocations | (1.9) | (1.4) | (1.2)
Public Service Company of New Hampshire | Pension Plan |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Service Cost | 8.9 | 8.2 | 7.1
Interest Cost | 14.5 | 19.4 | 24.0
Expected Return on Plan Assets | (47.5) | (44.7) | (40.7)
Actuarial Loss | 20.7 | 15.6 | 10.6
Prior Service Cost/(Credit) | 0.0 | 0.0 | 0.0
Total Net Periodic Benefit Plan Expense/(Income) | (3.4) | (1.5) | 1.0
Intercompany Expense/(Income) Allocations | 2.7 | 2.9 | 2.3
Public Service Company of New Hampshire | Other Postretirement Benefits Plan |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Service Cost | 1.2 | 0.9 | 0.7
Interest Cost | 1.8 | 2.8 | 3.4
Expected Return on Plan Assets | (6.1) | (5.7) | (5.4)
Actuarial Loss | 0.7 | 0.8 | 0.3
Prior Service Cost/(Credit) | 0.4 | 0.4 | 0.4
Total Net Periodic Benefit Plan Expense/(Income) | (2.0) | (0.8) | (0.6)
Intercompany Expense/(Income) Allocations | $ (0.6) | $ (0.5) | $ (0.4)

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