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Retirement Plans and Postretirement Benefit Plans (Tables) | 3 Months Ended
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Mar. 31, 2018
Defined Benefit Plan [Abstract] |
Schedule of Defined Benefit Plans Disclosures [Table Text Block] | The following table presents the Company's Pension Plan, Restoration of Retirement Income Plan and postretirement benefit plans components of net benefit cost, before consideration of capitalized amounts, grouped under the corresponding individual Condensed Consolidated Statements of Income line item.
| Pension Plan |  | Restoration of Retirement Income Plan |  | Postretirement Benefit Plans
| Three Months Ended |  | Three Months Ended |  | Three Months Ended
| March 31, |  | March 31, |  | March 31,
(In millions) | 2018 | 2017 |  | 2018 | 2017 |  | 2018 | 2017
Included in Other Operation and Maintenance: |  |  |  |  |  |  |  |
Service cost | $ | 4.1 |  | $ | 4.2 |  |  | $ | 0.1 |  | $ | 0.1 |  |  | $ | 0.1 |  | $ | 0.2 |
Included in Other Net Periodic Pension and Postretirement Benefit (Cost): |  |  |  |  |  |  |  |
Interest cost | 5.9 |  | 6.5 |  |  | 0.1 |  | 0.1 |  |  | 1.3 |  | 2.2 |
Expected return on plan assets | (11.3 | ) | (10.7 | ) |  | — |  | — |  |  | (0.5 | ) | (0.6 | )
Amortization of net loss | 3.9 |  | 4.0 |  |  | 0.1 |  | 0.1 |  |  | 1.0 |  | 0.6 |
Amortization of unrecognized prior service cost (A) | — |  | — |  |  | — |  | — |  |  | (2.1 | ) | — |
Total net periodic benefit cost | 2.6 |  | 4.0 |  |  | 0.3 |  | 0.3 |  |  | (0.2 | ) | 2.4 |
Less: Amount paid by unconsolidated affiliates | 0.5 |  | 0.8 |  |  | — |  | — |  |  | (0.1 | ) | 0.4 |
Net periodic benefit cost (net of unconsolidated affiliates) (B) | $ | 2.1 |  | $ | 3.2 |  |  | $ | 0.3 |  | $ | 0.3 |  |  | $ | (0.1 | ) | $ | 2.0 |
(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 $2.3 million and $5.5 million of net periodic benefit cost recognized during the three months ended March 31, 2018 and 2017, respectively, OG&E recognized the following:
• | an increase in pension expense during the three months ended March 31, 2018 and 2017 of $4.0 million and $2.9 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); and
• | an increase in postretirement medical expense in the three months ended March 31, 2018 and 2017 of $2.1 million and $1.1 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] |
| Three Months Ended
| March 31,
(In millions) | 2018 | 2017
Capitalized portion of net periodic pension benefit cost | $ | 1.0 |  | $ | 1.0 |
Capitalized portion of net periodic postretirement benefit cost | $ | — |  | $ | 0.7 |

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- Definition Tabular disclosure of one or more of the entity's defined benefit pension plans or one or more other defined benefit postretirement plans, separately for pension plans and other postretirement benefit plans including the entity's schedule of fair value of plan assets for defined benefit or other postretirement plans. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2439-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2417-114920 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2410-114920 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 55 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=77993564&loc=d3e4179-114921 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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