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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
--- | ---
Mar. 31, 2022 | Mar. 31, 2021
Pension and SERP |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Service Cost | $ 17.7 | $ 21.3
Interest Cost | 38.6 | 32.6
Expected Return on Plan Assets | (131.7) | (108.9)
Actuarial Loss | 30.7 | 61.6
Prior Service Cost/(Credit) | 0.4 | 0.3
Total Net Periodic Benefit Plan Expense/(Income) | (44.3) | 6.9
PBOP |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Service Cost | 2.8 | 3.5
Interest Cost | 5.0 | 4.4
Expected Return on Plan Assets | (22.4) | (19.7)
Actuarial Loss | 0.0 | 2.6
Prior Service Cost/(Credit) | (5.4) | (5.3)
Total Net Periodic Benefit Plan Expense/(Income) | (20.0) | (14.5)
CL&P | Pension and SERP |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Service Cost | 4.5 | 6.3
Interest Cost | 7.8 | 7.3
Expected Return on Plan Assets | (26.6) | (21.6)
Actuarial Loss | 4.2 | 13.0
Prior Service Cost/(Credit) | 0.0 | 0.0
Total Net Periodic Benefit Plan Expense/(Income) | (10.1) | 5.0
Intercompany Expense/(Income) Allocations | (3.8) | 1.3
CL&P | PBOP |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Service Cost | 0.5 | 0.6
Interest Cost | 0.9 | 0.8
Expected Return on Plan Assets | (2.8) | (2.6)
Actuarial Loss | 0.0 | 0.4
Prior Service Cost/(Credit) | 0.2 | 0.2
Total Net Periodic Benefit Plan Expense/(Income) | (1.2) | (0.6)
Intercompany Expense/(Income) Allocations | (0.9) | (0.3)
NSTAR Electric | Pension and SERP |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Service Cost | 3.6 | 4.0
Interest Cost | 8.1 | 6.7
Expected Return on Plan Assets | (32.0) | (26.9)
Actuarial Loss | 8.5 | 15.5
Prior Service Cost/(Credit) | 0.1 | 0.1
Total Net Periodic Benefit Plan Expense/(Income) | (11.7) | (0.6)
Intercompany Expense/(Income) Allocations | (2.9) | 1.5
NSTAR Electric | PBOP |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Service Cost | 0.5 | 0.6
Interest Cost | 1.3 | 1.1
Expected Return on Plan Assets | (10.6) | (9.3)
Actuarial Loss | 0.0 | 0.6
Prior Service Cost/(Credit) | (4.3) | (4.2)
Total Net Periodic Benefit Plan Expense/(Income) | (13.1) | (11.2)
Intercompany Expense/(Income) Allocations | (0.9) | (0.4)
PSNH | Pension and SERP |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Service Cost | 1.8 | 2.2
Interest Cost | 4.2 | 3.6
Expected Return on Plan Assets | (14.1) | (11.9)
Actuarial Loss | 2.2 | 4.9
Prior Service Cost/(Credit) | 0.0 | 0.0
Total Net Periodic Benefit Plan Expense/(Income) | (5.9) | (1.2)
Intercompany Expense/(Income) Allocations | (0.8) | 0.5
PSNH | PBOP |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Service Cost | 0.2 | 0.3
Interest Cost | 0.5 | 0.5
Expected Return on Plan Assets | (1.6) | (1.5)
Actuarial Loss | 0.0 | 0.3
Prior Service Cost/(Credit) | 0.1 | 0.1
Total Net Periodic Benefit Plan Expense/(Income) | (0.8) | (0.3)
Intercompany Expense/(Income) Allocations | $ (0.3) | $ (0.1)

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- Definition Amount represents allocation of pension and other post retirement benefits expense from parent and other that are not included in pension and post-retirement benefit expense. + References No definition available. + Details
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- Definition Amount of expected return (loss) recognized in net periodic benefit (cost) credit, calculated based on expected long-term rate of return and market-related value of plan assets of defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 55 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=123450688&loc=d3e4179-114921 Reference 2: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 55 -Paragraph 18 -URI http://asc.fasb.org/extlink&oid=123450688&loc=d3e4587-114921 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(3) -URI http://asc.fasb.org/extlink&oid=123447040&loc=d3e1928-114920 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 45 -Paragraph 3A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=123453770&loc=SL108413299-114919 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 6 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=123447040&loc=d3e2709-114920 + Details
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