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Summary of Significant Accounting Policies (Tables) | 9 Months Ended
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Sep. 30, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract] |
Schedule of Regulatory Assets and Liabilities [Table Text Block] | The following table is a summary of OG&E's regulatory assets and liabilities.
| September 30, | December 31,
(In millions) | 2018 | 2017
REGULATORY ASSETS |  |
Current: |  |
Oklahoma demand program rider under recovery (A) | $ | 10.7 |  | $ | 31.6 |
Production tax credit rider under recovery (A) | 6.1 |  | — |
SPP cost tracker under recovery (A) | — |  | 7.7 |
Other (A) | 1.6 |  | 1.5 |
Total current regulatory assets | $ | 18.4 |  | $ | 40.8 |
Non-current: |  |  |  |
Benefit obligations regulatory asset | $ | 160.8 |  | $ | 177.2 |
Deferred storm expenses | 37.4 |  | 42.2 |
Smart Grid | 27.5 |  | 32.8 |
Unamortized loss on reacquired debt | 11.6 |  | 12.3 |
Other | 18.9 |  | 18.5 |
Total non-current regulatory assets | $ | 256.2 |  | $ | 283.0 |
REGULATORY LIABILITIES |  |  |  |
Current: |  |  |  |
Fuel clause over recoveries | $ | 39.2 |  | $ | 1.7 |
SPP cost tracker over recovery (B) | 9.4 |  | — |
Other (B) | 3.5 |  | 2.2 |
Total current regulatory liabilities | $ | 52.1 |  | $ | 3.9 |
Non-current: |  |  |  |
Income taxes refundable to customers, net | $ | 943.9 |  | $ | 955.5 |
Accrued removal obligations, net | 308.2 |  | 288.4 |
Pension tracker | 32.0 |  | 32.3 |
Other | 6.9 |  | 7.2 |
Total non-current regulatory liabilities | $ | 1,291.0 |  | $ | 1,283.4 |
(A) | Included in Other Current Assets on the Condensed Consolidated Balance Sheets.
(B) | Included in Other Current Liabilities on the Condensed Consolidated Balance Sheets.
As discussed in Note 14 under "Oklahoma Rate Review Filing - 2018," as a result of the settlement agreement reached in the most recent Oklahoma rate review, OG&E removed production tax credits from base rates and now utilizes a separate rider to credit customers the differential between estimated and actual production tax credits, which can either result in a regulatory asset or regulatory liability based on that differential.
Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block] | The following tables summarize changes in the components of accumulated other comprehensive income (loss) attributable to the Company during the nine months ended September 30, 2018 and 2017. All amounts below are presented net of tax.
| Pension Plan and Restoration of Retirement Income Plan |  | Postretirement Benefit Plans |
(In millions) | Net Income (Loss) | Prior Service Cost |  | Net Income | Prior Service Credit | Total
Balance at December 31, 2017 | $ | (32.7 | ) | $ | — |  |  | $ | 2.5 |  | $ | 7.0 |  | $ | (23.2 | )
Amounts reclassified from accumulated other comprehensive income (loss) | 2.5 |  | — |  |  | — |  | (1.3 | ) | 1.2 |
Settlement cost | 3.1 |  | — |  |  | — |  | — |  | 3.1 |
Balance at September 30, 2018 | $ | (27.1 | ) | $ | — |  |  | $ | 2.5 |  | $ | 5.7 |  | $ | (18.9 | )
| Pension Plan and Restoration of Retirement Income Plan |  | Postretirement Benefit Plans |
(In millions) | Net Income (Loss) | Prior Service Cost |  | Net Income | Prior Service Cost (Credit) | Total
Balance at December 31, 2016 | $ | (32.1 | ) | $ | 0.1 |  |  | $ | 2.7 |  | $ | — |  | $ | (29.3 | )
Other comprehensive income before reclassifications | — |  | — |  |  | — |  | 6.7 |  | 6.7 |
Amounts reclassified from accumulated other comprehensive income (loss) | 2.0 |  | — |  |  | — |  | (0.2 | ) | 1.8 |
Settlement cost | — |  | — |  |  | 0.5 |  | — |  | 0.5 |
Net current period other comprehensive income | 2.0 |  | — |  |  | 0.5 |  | 6.5 |  | 9.0 |
Balance at September 30, 2017 | $ | (30.1 | ) | $ | 0.1 |  |  | $ | 3.2 |  | $ | 6.5 |  | $ | (20.3 | )
Schedule of Amounts Reclassified out of Accumulated Other Comprehensive Income [Table Text Block] | The following table summarizes significant amounts reclassified out of accumulated other comprehensive income (loss) by the respective line items in net income during the three and nine months ended September 30, 2018 and 2017.
Details about Accumulated Other Comprehensive Income (Loss) Components | Amount Reclassified from Accumulated Other Comprehensive Income (Loss) | Affected Line Item in the Condensed Consolidated Statements of Income
| Three Months Ended | Nine Months Ended |
| September 30, | September 30, |
(In millions) | 2018 | 2017 | 2018 | 2017 |
Amortization of Pension Plan and Restoration of Retirement Income Plan items: |  |  |  |  |
Actuarial losses (A) | $ | (1.1 | ) | $ | (1.1 | ) | $ | (3.3 | ) | $ | (3.3 | ) | Other Net Periodic Benefit Income (Expense)
Settlement cost (A) | (4.2 | ) | — |  | (4.2 | ) | — |  | Other Net Periodic Benefit Income (Expense)
| (5.3 | ) | (1.1 | ) | (7.5 | ) | (3.3 | ) | Income Before Taxes
| (1.4 | ) | (0.5 | ) | (1.9 | ) | (1.3 | ) | Income Tax Expense
| $ | (3.9 | ) | $ | (0.6 | ) | $ | (5.6 | ) | $ | (2.0 | ) | Net Income
Amortization of postretirement benefit plans items: |  |  |  |  |
Prior service credit (A) | $ | 0.6 |  | $ | 0.4 |  | $ | 1.7 |  | $ | 0.4 |  | Other Net Periodic Benefit Income (Expense)
Settlement cost (A) | — |  | (0.7 | ) | — |  | (0.7 | ) | Other Net Periodic Benefit Income (Expense)
| 0.6 |  | (0.3 | ) | 1.7 |  | (0.3 | ) | Income Before Taxes
| 0.1 |  | — |  | 0.4 |  | — |  | Income Tax Expense
| $ | 0.5 |  | $ | (0.3 | ) | $ | 1.3 |  | $ | (0.3 | ) | Net Income
Total reclassifications for the period, net of tax | $ | (3.4 | ) | $ | (0.9 | ) | $ | (4.3 | ) | $ | (2.3 | ) | Net Income
(A) | These accumulated other comprehensive income (loss) components are included in the computation of net periodic benefit cost (see Note 11 for additional information).

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