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Retirement Plans and Postretirement Benefit Plans (Tables) | 9 Months Ended
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Sep. 30, 2018
Defined Benefit Plan [Abstract] |
Schedule of Defined Benefit Plans Disclosures [Table Text Block] | The following tables present the net periodic benefit cost components, before consideration of capitalized amounts, of the Company's Pension Plan, Restoration of Retirement Income Plan and postretirement benefit plans that are included in the Condensed Consolidated Financial Statements. Service cost is presented within Other Operation and Maintenance, and interest cost, expected return on plan assets, amortization of net loss, amortization of unrecognized prior service cost and settlement cost are presented within Other Net Periodic Benefit Income (Expense) on the Company's Condensed Consolidated Statements of Income. OG&E recovers specific amounts of pension and postretirement medical costs in rates approved in its Oklahoma rate reviews. In accordance with approved orders, OG&E defers the difference between actual pension and postretirement medical expenses and the amount approved in its last Oklahoma rate review as a regulatory asset or regulatory liability. These amounts have been recorded in the Pension tracker in the regulatory assets and liabilities table in Note 1 and within Other Net Periodic Benefit Income (Expense) on the Company's Condensed Consolidated Statements of Income.
| Pension Plan |  | Restoration of Retirement Income Plan
| Three Months Ended | Nine Months Ended |  | Three Months Ended | Nine Months Ended
| September 30, | September 30, |  | September 30, | September 30,
(In millions) | 2018 (B) | 2017 (B) | 2018 (C) | 2017 (C) |  | 2018 (B) | 2017 (B) | 2018 (C) | 2017 (C)
Service cost | $ | 3.6 |  | $ | 3.9 |  | $ | 11.1 |  | $ | 11.6 |  |  | $ | 0.1 |  | $ | — |  | $ | 0.3 |  | $ | 0.2 |
Interest cost | 6.1 |  | 6.5 |  | 17.8 |  | 19.6 |  |  | 0.1 |  | 0.1 |  | 0.3 |  | 0.2 |
Expected return on plan assets | (10.7 | ) | (10.7 | ) | (33.0 | ) | (32.0 | ) |  | — |  | — |  | — |  | — |
Amortization of net loss | 3.9 |  | 4.4 |  | 12.2 |  | 13.1 |  |  | 0.1 |  | 0.1 |  | 0.5 |  | 0.3 |
Amortization of unrecognized prior service cost (A) | — |  | — |  | — |  | — |  |  | 0.1 |  | 0.1 |  | 0.1 |  | 0.1 |
Settlement cost | 12.2 |  | — |  | 12.2 |  | — |  |  | 0.6 |  | — |  | 0.6 |  | — |
Total net periodic benefit cost | 15.1 |  | 4.1 |  | 20.3 |  | 12.3 |  |  | 1.0 |  | 0.3 |  | 1.8 |  | 0.8 |
Less: Amount paid by unconsolidated affiliates | 0.7 |  | 1.2 |  | 1.9 |  | 2.9 |  |  | 0.1 |  | — |  | 0.1 |  | — |
Net periodic benefit cost | $ | 14.4 |  | $ | 2.9 |  | $ | 18.4 |  | $ | 9.4 |  |  | $ | 0.9 |  | $ | 0.3 |  | $ | 1.7 |  | $ | 0.8 |
(A) | 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.
(B) | In addition to the $15.3 million and $3.2 million of net periodic benefit cost recognized during the three months ended September 30, 2018 and 2017, respectively, the Company recognized the following:
• | a deferral of pension expense during the three months ended September 30, 2018 of $10.5 million related to the pension settlement charge of $12.8 million in accordance with the Oklahoma Pension tracker regulatory liability (see Note 1);
• | a deferral of pension expense during the three months ended September 30, 2018 of $1.0 million related to the Arkansas jurisdictional portion of the pension settlement charge of $12.8 million; and
• | an increase in pension expense during the three months ended September 30, 2017 of $2.7 million to maintain the allowable amount to be recovered for pension expense in the Oklahoma jurisdiction, which is included in the Pension tracker regulatory liability (see Note 1).
(C) | In addition to the $20.1 million and $10.2 million of net periodic benefit cost recognized during the nine months ended September 30, 2018 and 2017, respectively, the Company recognized the following:
• | an increase in pension expense during the nine months ended September 30, 2018 and 2017 of $7.8 million and $8.5 million, respectively, to maintain the allowable amount to be recovered for pension expense in the Oklahoma jurisdiction, which are included in the Pension tracker regulatory liability (see Note 1);
• | a deferral of pension expense during the nine months ended September 30, 2018 of $10.5 million related to the pension settlement charge of $12.8 million in accordance with the Oklahoma Pension tracker regulatory liability (see Note 1);
• | a deferral of pension expense during the nine months ended September 30, 2018 of $1.0 million related to the Arkansas jurisdictional portion of the pension settlement charge of $12.8 million; and
• | a deferral of pension expense during the nine months ended September 30, 2017 of $2.3 million related to the Arkansas jurisdictional portion of the pension settlement charge of $22.4 million in 2013.
| Postretirement Benefit Plans
| Three Months Ended | Nine Months Ended
| September 30, | September 30,
(In millions) | 2018 (B) | 2017 (B) | 2018 (C) | 2017 (C)
Service cost | $ | — |  | $ | 0.1 |  | $ | 0.2 |  | $ | 0.5 |
Interest cost | 1.5 |  | 1.6 |  | 4.1 |  | 5.9 |
Expected return on plan assets | (0.5 | ) | (0.6 | ) | (1.5 | ) | (1.7 | )
Amortization of net loss | 1.0 |  | 0.5 |  | 2.9 |  | 1.3 |
Amortization of unrecognized prior service cost (A) | (2.1 | ) | (1.4 | ) | (6.3 | ) | (1.4 | )
Settlement cost | — |  | 0.6 |  | — |  | 0.6 |
Total net periodic benefit cost | (0.1 | ) | 0.8 |  | (0.6 | ) | 5.2 |
Less: Amount paid by unconsolidated affiliates | (0.2 | ) | (0.1 | ) | (0.4 | ) | 0.5 |
Net periodic benefit cost | $ | 0.1 |  | $ | 0.9 |  | $ | (0.2 | ) | $ | 4.7 |
(A) | 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.
(B) | In addition to the $0.1 million and $0.9 million of net periodic benefit cost recognized during the three months ended September 30, 2018 and 2017, respectively, the Company recognized an increase in postretirement medical expense in the three months ended September 30, 2018 and 2017 of $0.1 million and $1.9 million, respectively, to maintain the allowable amount to be recovered for postretirement medical expense in the Oklahoma jurisdiction which are included in the Pension tracker regulatory liability (see Note 1).
(C) | In addition to the $0.2 million of net periodic benefit income and $4.7 million of net periodic benefit cost recognized during the nine months ended September 30, 2018 and 2017, respectively, the Company recognized an increase in postretirement medical expense in the nine months ended September 30, 2018 and 2017 of $4.3 million and $3.9 million, respectively, to maintain the allowable amount to be recovered for postretirement medical expense in the Oklahoma jurisdiction which are included in the Pension tracker regulatory liability (see Note 1).
Schedule of Capitalized Pension and Postretirement Cost [Table Text Block] | As required by ASU 2017-07, the Company only capitalizes the service cost component of net benefit cost, beginning in the first quarter of 2018. Prior year capitalized amounts were not adjusted, as this change was implemented on a prospective basis.
| Three Months Ended | Nine Months Ended
| September 30, | September 30,
(In millions) | 2018 | 2017 | 2018 | 2017
Capitalized portion of net periodic pension benefit cost | $ | 0.9 |  | $ | 1.0 |  | $ | 2.8 |  | $ | 3.3 |
Capitalized portion of net periodic postretirement benefit cost | $ | — |  | $ | 0.1 |  | $ | 0.1 |  | $ | 1.3 |

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