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Retirement Plans and Postretirement Benefit Plans (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2021 | Jun. 30, 2020 | Jun. 30, 2021 | Jun. 30, 2020
Defined Benefit Plan, Plan Assets, Contributions by Employer |  |  |  | $ 40.0 |
Pension Plan [Member] |  |  |  |  |
Other Comprehensive Income (Loss), Defined Benefit Plan, Adjustment for Settlement or Curtailment Gain (Loss), Tax |  | $ 0.6 | $ 0.1 | 1.6 | $ 0.1
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract] |  |  |  |  |
Service cost |  | 2.6 | 2.8 | 5.6 | 6.5
Interest cost |  | 3.4 | 4.6 | 6.9 | 9.2
Expected return on plan assets |  | (8.7) | (9.2) | (17.1) | (18.6)
Amortization of net loss |  | 1.9 | 4.6 | 4.7 | 8.4
Settlement cost |  | 5.3 | 0.0 | 31.7 | 0.0
Total net periodic benefit cost |  | 4.5 | 2.8 | 31.8 | 5.5
Amount Attributable to Unconsolidated Affiliates | [1] | (0.2) | 0.6 | (0.1) | 1.1
Net periodic benefit cost |  | 4.7 | 2.2 | 31.9 | 4.4
Capitalized portion of net periodic pension benefit cost |  | 0.8 | 0.8 | 1.6 | 1.8
Other Pension Plan [Member] |  |  |  |  |
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract] |  |  |  |  |
Service cost |  | 0.2 | 0.2 | 0.3 | 0.4
Interest cost |  | 0.1 | 0.0 | 0.1 | 0.1
Expected return on plan assets |  | 0.0 | 0.0 | 0.0 | 0.0
Amortization of net loss |  | 0.0 | 0.2 | 0.1 | 0.3
Settlement cost |  | 0.4 | 0.3 | 0.4 | 0.3
Total net periodic benefit cost |  | 0.7 | 0.7 | 0.9 | 1.1
Amount Attributable to Unconsolidated Affiliates | [1] | 0.0 | 0.0 | 0.0 | 0.0
Net periodic benefit cost |  | 0.7 | 0.7 | 0.9 | 1.1
Postretirement Benefit Plan [Member] |  |  |  |  |
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract] |  |  |  |  |
Service cost |  | 0.0 | 0.0 | 0.1 | 0.1
Interest cost |  | 0.9 | 1.0 | 1.7 | 2.1
Expected return on plan assets |  | (0.5) | (0.4) | (1.0) | (0.9)
Amortization of net loss |  | 0.7 | 0.4 | 1.4 | 1.0
Defined Benefit Plan, Amortization of Prior Service Cost (Credit) | [2] | (1.7) | (2.1) | (3.4) | (4.2)
Total net periodic benefit cost |  | (0.6) | (1.1) | (1.2) | (1.9)
Amount Attributable to Unconsolidated Affiliates | [1] | (0.2) | (0.2) | (0.3) | (0.4)
Net periodic benefit cost |  | (0.4) | (0.9) | (0.9) | (1.5)
Capitalized portion of net periodic pension benefit cost |  | 0.0 | 0.1 | 0.1 | 0.1
OKLAHOMA | Pension Plan [Member] |  |  |  |  |
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract] |  |  |  |  |
Settlement cost | [3] | 5.2 | 0.3 | 28.9 | 0.3
Additional Pension Expense to Meet State Requirements | [3] | 4.2 | 1.6 | 7.2 | 3.3
OKLAHOMA | Postretirement Benefit Plan [Member] |  |  |  |  |
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract] |  |  |  |  |
Additional Pension Expense to Meet State Requirements | [4] | 0.1 | 0.6 | 0.2 | 0.8
ARKANSAS | Pension Plan [Member] |  |  |  |  |
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract] |  |  |  |  |
Settlement cost | [3] | 0.5 | 0.0 | 2.7 | 0.0
OG&E [Member] |  |  |  |  |
Defined Benefit Plan, Plan Assets, Contributions by Employer |  |  |  | 30.0 |
OG&E [Member] | Pension Plan [Member] |  |  |  |  |
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract] |  |  |  |  |
Service cost |  | 1.7 | 1.9 | 3.8 | 4.5
Interest cost |  | 2.5 | 3.4 | 5.0 | 6.8
Expected return on plan assets |  | (6.2) | (6.8) | (12.3) | (13.9)
Amortization of net loss |  | 1.5 | 3.2 | 3.5 | 6.0
Settlement cost |  | 4.4 | 0.0 | 26.6 | 0.0
Total net periodic benefit cost |  | 3.9 | 1.7 | 26.6 | 3.4
Amount attributable to parent | [1] | 0.7 | 0.5 | 4.6 | 1.0
Net periodic benefit cost |  | 4.6 | 2.2 | 31.2 | 4.4
Capitalized portion of net periodic pension benefit cost |  | 0.7 | 0.6 | 1.4 | 1.5
OG&E [Member] | Other Pension Plan [Member] |  |  |  |  |
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract] |  |  |  |  |
Service cost |  | 0.0 | 0.0 | 0.0 | 0.1
Interest cost |  | 0.0 | 0.1 | 0.0 | 0.1
Expected return on plan assets |  | 0.0 | 0.0 | 0.0 | 0.0
Amortization of net loss |  | 0.1 | 0.1 | 0.1 | 0.2
Settlement cost |  | 0.3 | 0.0 | 0.3 | 0.0
Total net periodic benefit cost |  | 0.4 | 0.2 | 0.4 | 0.4
Amount attributable to parent | [1] | 0.3 | 0.4 | 0.5 | 0.6
Net periodic benefit cost |  | 0.7 | 0.6 | 0.9 | 1.0
OG&E [Member] | Postretirement Benefit Plan [Member] |  |  |  |  |
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract] |  |  |  |  |
Service cost |  | 0.0 | 0.1 | 0.1 | 0.1
Interest cost |  | 0.7 | 0.8 | 1.3 | 1.6
Expected return on plan assets |  | (0.5) | (0.4) | (0.9) | (0.8)
Amortization of net loss |  | 0.7 | 0.4 | 1.4 | 1.1
Defined Benefit Plan, Amortization of Prior Service Cost (Credit) | [2] | (1.2) | (1.6) | (2.5) | (3.1)
Total net periodic benefit cost |  | (0.3) | (0.7) | (0.6) | (1.1)
Amount attributable to parent | [1] | (0.1) | (0.2) | (0.2) | (0.4)
Net periodic benefit cost |  | (0.4) | (0.9) | (0.8) | (1.5)
Capitalized portion of net periodic pension benefit cost |  | $ 0.1 | $ 0.1 | $ 0.1 | $ 0.1
[1] | "Amount paid by unconsolidated affiliates" is only applicable to OGE Energy. "Amount allocated from OGE Energy" is only applicable to OG&E.
[2] | Unamortized prior service cost is amortized on a straight-line basis over the average remaining service period to the first eligibility age of participants who are expected to receive a benefit and are active at the date of the plan amendment.
[3] | Included in the pension regulatory asset or liability in each jurisdiction, as indicated in the regulatory assets and liabilities table in Note 1.
[4] | Included in the Pension tracker, as presented in the regulatory assets and liabilities table in Note 1.

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