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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, 2021 | Sep. 30, 2020 | Sep. 30, 2021 | Sep. 30, 2020
Pension and SERP |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Service Cost | $ 21.5 | $ 18.7 | $ 64.3 | $ 57.0
Interest Cost | 32.5 | 44.4 | 97.5 | 133.0
Expected Return on Plan Assets | (109.5) | (99.2) | (328.0) | (298.8)
Actuarial Loss | 60.7 | 50.7 | 183.1 | 150.7
Prior Service Cost/(Credit) | 0.3 | 0.2 | 1.1 | 0.8
Total Net Periodic Benefit Plan Expense/(Income) | 5.5 | 14.8 | 18.0 | 42.7
PBOP |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Service Cost | 3.3 | 2.4 | 10.1 | 7.3
Interest Cost | 4.3 | 6.1 | 12.9 | 18.3
Expected Return on Plan Assets | (19.8) | (18.3) | (59.3) | (55.1)
Actuarial Loss | 2.0 | 2.2 | 5.9 | 6.3
Prior Service Cost/(Credit) | (5.3) | (5.3) | (15.9) | (15.9)
Total Net Periodic Benefit Plan Expense/(Income) | (15.5) | (12.9) | (46.3) | (39.1)
CL&P | Pension and SERP |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Service Cost | 5.6 | 5.4 | 17.4 | 16.5
Interest Cost | 6.7 | 9.3 | 20.7 | 28.1
Expected Return on Plan Assets | (21.7) | (19.8) | (65.0) | (59.6)
Actuarial Loss | 10.8 | 9.7 | 34.6 | 29.4
Prior Service Cost/(Credit) | 0.0 | 0.0 | 0.0 | 0.0
Total Net Periodic Benefit Plan Expense/(Income) | 1.4 | 4.6 | 7.7 | 14.4
Intercompany Expense/(Income) Allocations | 2.2 | 2.4 | 5.8 | 6.7
CL&P | PBOP |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Service Cost | 0.5 | 0.4 | 1.7 | 1.3
Interest Cost | 0.8 | 1.1 | 2.4 | 3.3
Expected Return on Plan Assets | (2.5) | (2.5) | (7.7) | (7.4)
Actuarial Loss | 0.4 | 0.3 | 1.2 | 0.9
Prior Service Cost/(Credit) | 0.3 | 0.3 | 0.8 | 0.8
Total Net Periodic Benefit Plan Expense/(Income) | (0.5) | (0.4) | (1.6) | (1.1)
Intercompany Expense/(Income) Allocations | (0.4) | (0.3) | (1.3) | (0.9)
NSTAR Electric | Pension and SERP |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Service Cost | 4.0 | 3.8 | 11.9 | 11.4
Interest Cost | 6.7 | 9.7 | 20.1 | 29.0
Expected Return on Plan Assets | (27.1) | (25.7) | (81.1) | (77.2)
Actuarial Loss | 15.3 | 14.0 | 46.1 | 41.3
Prior Service Cost/(Credit) | 0.1 | 0.0 | 0.3 | 0.2
Total Net Periodic Benefit Plan Expense/(Income) | (1.0) | 1.8 | (2.7) | 4.7
Intercompany Expense/(Income) Allocations | 2.4 | 2.3 | 6.4 | 6.5
NSTAR Electric | PBOP |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Service Cost | 0.6 | 0.5 | 1.8 | 1.6
Interest Cost | 1.1 | 1.7 | 3.3 | 5.0
Expected Return on Plan Assets | (9.2) | (8.5) | (27.7) | (25.5)
Actuarial Loss | 0.5 | 0.6 | 1.6 | 1.8
Prior Service Cost/(Credit) | (4.3) | (4.2) | (12.7) | (12.7)
Total Net Periodic Benefit Plan Expense/(Income) | (11.3) | (9.9) | (33.7) | (29.8)
Intercompany Expense/(Income) Allocations | (0.5) | (0.4) | (1.5) | (1.0)
PSNH | Pension and SERP |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Service Cost | 2.2 | 2.0 | 6.6 | 6.2
Interest Cost | 3.7 | 4.8 | 10.9 | 14.5
Expected Return on Plan Assets | (11.9) | (11.1) | (35.6) | (33.5)
Actuarial Loss | 5.3 | 3.8 | 15.4 | 11.8
Prior Service Cost/(Credit) | 0.0 | 0.0 | 0.0 | 0.0
Total Net Periodic Benefit Plan Expense/(Income) | (0.7) | (0.5) | (2.7) | (1.0)
Intercompany Expense/(Income) Allocations | 0.7 | 0.8 | 1.9 | 2.2
PSNH | PBOP |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Service Cost | 0.3 | 0.2 | 0.9 | 0.6
Interest Cost | 0.4 | 0.7 | 1.3 | 2.1
Expected Return on Plan Assets | (1.6) | (1.4) | (4.6) | (4.2)
Actuarial Loss | 0.1 | 0.2 | 0.4 | 0.6
Prior Service Cost/(Credit) | 0.1 | 0.1 | 0.3 | 0.3
Total Net Periodic Benefit Plan Expense/(Income) | (0.7) | (0.2) | (1.7) | (0.6)
Intercompany Expense/(Income) Allocations | $ (0.2) | $ (0.1) | $ (0.5) | $ (0.4)

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