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Summary of Significant Accounting Policies (Tables) | 3 Months Ended
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Mar. 31, 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.
| March 31, | December 31,
(In millions) | 2018 | 2017
Regulatory Assets |  |
Current: |  |
Oklahoma demand program rider under recovery (A) | $ | 23.9 |  | $ | 31.6 |
SPP cost tracker under recovery (A) | 6.9 |  | 7.7 |
Other (A) | 0.8 |  | 1.5 |
Total current regulatory assets | $ | 31.6 |  | $ | 40.8 |
Non-current: |  |  |  |
Benefit obligations regulatory asset | $ | 174.6 |  | $ | 177.2 |
Deferred storm expenses | 39.7 |  | 42.2 |
Smart Grid | 31.0 |  | 32.8 |
Unamortized loss on reacquired debt | 12.1 |  | 12.3 |
Other | 18.2 |  | 18.5 |
Total non-current regulatory assets | $ | 275.6 |  | $ | 283.0 |
Regulatory Liabilities |  |  |  |
Current: |  |  |  |
Fuel clause over recoveries | $ | 49.9 |  | $ | 1.7 |
Other (B) | 2.1 |  | 2.2 |
Total current regulatory liabilities | $ | 52.0 |  | $ | 3.9 |
Non-current: |  |  |  |
Income taxes refundable to customers, net | $ | 951.3 |  | $ | 955.5 |
Accrued removal obligations, net | 295.7 |  | 288.4 |
Pension tracker | 38.4 |  | 32.3 |
Other | 7.3 |  | 7.2 |
Total non-current regulatory liabilities | $ | 1,292.7 |  | $ | 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.
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 three months ended March 31, 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 cost | Total
Balance at December 31, 2017 | $ | (32.7 | ) | $ | — |  |  | $ | 2.5 |  | $ | 7.0 |  | $ | (23.2 | )
Amounts reclassified from accumulated other comprehensive income (loss) | 0.7 |  | — |  |  | — |  | (0.5 | ) | 0.2 |
Balance at March 31, 2018 | $ | (32.0 | ) | $ | — |  |  | $ | 2.5 |  | $ | 6.5 |  | $ | (23.0 | )
| Pension Plan and Restoration of Retirement Income Plan |  | Postretirement Benefit Plans |
(In millions) | Net income (loss) | Prior service cost |  | Net income | Prior service cost | Total
Balance at December 31, 2016 | $ | (32.1 | ) | $ | 0.1 |  |  | $ | 2.7 |  | $ | — |  | $ | (29.3 | )
Amounts reclassified from accumulated other comprehensive income | 0.6 |  | — |  |  | — |  | — |  | 0.6 |
Balance at March 31, 2017 | $ | (31.5 | ) | $ | 0.1 |  |  | $ | 2.7 |  | $ | — |  | $ | (28.7 | )
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 months ended March 31, 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 Comprehensive Income
| Three Months Ended |
| March 31, |
(In millions) | 2018 | 2017 |
Amortization of Pension Plan and Restoration of Retirement Income Plan items: |  |  |
Actuarial losses (A) | $ | (0.9 | ) | $ | (1.0 | ) | Other Net Periodic Pension and Postretirement Benefit (Cost)
| (0.2 | ) | (0.4 | ) | Income Tax Expense
| $ | (0.7 | ) | $ | (0.6 | ) | Net Income
Amortization of postretirement benefit plan items: |  |  |
Prior service credit (A) | $ | 0.6 |  | $ | — |  | Other Net Periodic Pension and Postretirement Benefit (Cost)
| 0.1 |  | — |  | Income Tax Expense
| $ | 0.5 |  | $ | — |  | Net Income
Total reclassifications for the period, net of tax | $ | (0.2 | ) | $ | (0.6 | ) | 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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- Definition A table of assets and liabilities that are created when regulatory agencies permit public utilities to defer certain costs and recognition of certain revenues that are included in rate-setting to the balance sheet. + References No definition available. + Details
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- Definition Tabular disclosure of the components of accumulated other comprehensive income (loss). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -URI http://asc.fasb.org/extlink&oid=84228862&loc=SL7669686-108580 + Details
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