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Summary of Significant Accounting Policies (Tables) | 12 Months Ended
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Dec. 31, 2017
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:
December 31 (In millions) | 2017 | 2016
Regulatory Assets |  |
Current: |  |
Oklahoma demand program rider under recovery (A) | $ | 31.6 |  | $ | 51.0 |
SPP cost tracker under recovery (A) | 7.7 |  | 10.0 |
Fuel clause under recoveries | — |  | 51.3 |
Other (A) | 1.5 |  | 9.5 |
Total current regulatory assets | $ | 40.8 |  | $ | 121.8 |
Non-current: |  |
Benefit obligations regulatory asset | $ | 177.2 |  | $ | 232.6 |
Deferred storm expenses | 42.2 |  | 35.7 |
Smart Grid | 32.8 |  | 43.2 |
Unamortized loss on reacquired debt | 12.3 |  | 13.4 |
Income taxes recoverable from customers, net | — |  | 62.3 |
Other | 18.5 |  | 17.6 |
Total non-current regulatory assets | $ | 283.0 |  | $ | 404.8 |
Regulatory Liabilities |  |
Current: |  |
Fuel clause over recoveries | $ | 1.7 |  | $ | — |
Other (B) | 2.2 |  | 12.3 |
Total current regulatory liabilities | $ | 3.9 |  | $ | 12.3 |
Non-current: |  |
Income taxes refundable to customers, net | $ | 955.5 |  | $ | — |
Accrued removal obligations, net | 288.4 |  | 262.8 |
Pension tracker | 32.3 |  | 35.5 |
Other | 7.2 |  | 1.4 |
Total non-current regulatory liabilities | $ | 1,283.4 |  | $ | 299.7 |
(A) | Included in Other Current Assets on the Consolidated Balance Sheets.
(B) | Included in Other Current Liabilities on the Consolidated Balance Sheets.
Components of Benefit Obligation Regulatory Asset [Table Text Block] | The following table is a summary of the components of the benefit obligations regulatory asset:
December 31 (In millions) | 2017 | 2016
Pension Plan and Restoration of Retirement Income Plan: |  |
Net loss | $ | 172.4 |  | $ | 199.9 |
Postretirement Benefit Plans: |  |
Net loss | 33.6 |  | 32.7 |
Prior service cost | (28.8 | ) | — |
Total | $ | 177.2 |  | $ | 232.6 |
Schedule of Net Periodic Benefit Cost Not yet Recognized [Table Text Block] | The following amounts in the benefit obligations regulatory asset at December 31, 2017 are expected to be recognized as components of net periodic benefit cost in 2018:
(In millions) |
Pension Plan and Restoration of Retirement Income Plan: |
Net loss | $ | 12.5 |
Postretirement Benefit Plans: |
Net loss | 4.0 |
Prior service cost | (6.1 | )
Total | $ | 10.4 |
Schedule of Jointly Owned Utility Plants [Table Text Block] | The tables below present OG&E's ownership interest in the jointly-owned McClain Plant and the jointly-owned Redbud Plant, and, as disclosed below, only OG&E's ownership interest is reflected in the property, plant and equipment and accumulated depreciation balances in these tables. The owners of the remaining interests in the McClain Plant and the Redbud Plant are responsible for providing their own financing of capital expenditures. Also, only OG&E's proportionate interests of any direct expenses of the McClain Plant and the Redbud Plant, such as fuel, maintenance expense and other operating expenses, are included in the applicable financial statement captions in the Consolidated Statements of Income.
December 31, 2017 (In millions) | Percentage Ownership | Total Property, Plant and Equipment | Accumulated Depreciation | Net Property, Plant and Equipment
McClain Plant (A) | 77 | % | $ | 226.8 |  | $ | 71.4 |  | $ | 155.4 |
Redbud Plant (A)(B) | 51 | % | $ | 496.6 |  | $ | 136.0 |  | $ | 360.6 |
(A) | Construction work in progress was $0.4 million and $7.8 million for the McClain and Redbud Plants, respectively.
(B) | This amount includes a plant acquisition adjustment of $148.3 million and accumulated amortization of $50.8 million.
December 31, 2016 (In millions) | Percentage Ownership | Total Property, Plant and Equipment | Accumulated Depreciation | Net Property, Plant and Equipment
McClain Plant (A) | 77 | % | $ | 234.2 |  | $ | 72.3 |  | $ | 161.9 |
Redbud Plant (A)(B) | 51 | % | $ | 489.0 |  | $ | 121.0 |  | $ | 368.0 |
(A) | Construction work in progress was $0.2 million and $1.8 million for the McClain and Redbud Plants, respectively.
(B) | This amount includes a plant acquisition adjustment of $148.3 million and accumulated amortization of $45.3 million.
Property, Plant and Equipment [Table Text Block] | The Company's property, plant and equipment and related accumulated depreciation are divided into the following major classes:
December 31, 2017 (In millions) | Total Property, Plant and Equipment | Accumulated Depreciation | Net Property, Plant and Equipment
OGE Energy (holding company): |  |  |
Property, plant and equipment | $ | 6.1 |  | $ | — |  | $ | 6.1 |
OGE Energy property, plant and equipment | 6.1 |  | — |  | 6.1 |
OG&E: |  |  |
Distribution assets | 4,057.1 |  | 1,259.1 |  | 2,798.0 |
Electric generation assets (A) | 4,475.0 |  | 1,493.5 |  | 2,981.5 |
Transmission assets (B) | 2,767.7 |  | 506.5 |  | 2,261.2 |
Intangible plant | 181.8 |  | 135.8 |  | 46.0 |
Other property and equipment | 421.0 |  | 173.9 |  | 247.1 |
OG&E property, plant and equipment | 11,902.6 |  | 3,568.8 |  | 8,333.8 |
Total property, plant and equipment | $ | 11,908.7 |  | $ | 3,568.8 |  | $ | 8,339.9 |
(A) | This amount includes a plant acquisition adjustment of $148.3 million and accumulated amortization of $50.8 million.
(B) | This amount includes a plant acquisition adjustment of $3.3 million and accumulated amortization of $0.6 million.
December 31, 2016 (In millions) | Total Property, Plant and Equipment | Accumulated Depreciation | Net Property, Plant and Equipment
OGE Energy (holding company): |  |  |
Property, plant and equipment | $ | 117.7 |  | $ | 103.3 |  | $ | 14.4 |
OGE Energy property, plant and equipment | 117.7 |  | 103.3 |  | 14.4 |
OG&E: |  |  |
Distribution assets | 3,896.2 |  | 1,221.5 |  | 2,674.7 |
Electric generation assets (A) | 4,155.9 |  | 1,493.3 |  | 2,662.6 |
Transmission assets (B) | 2,548.8 |  | 481.3 |  | 2,067.5 |
Intangible plant | 85.0 |  | 43.9 |  | 41.1 |
Other property and equipment | 381.5 |  | 145.6 |  | 235.9 |
OG&E property, plant and equipment | 11,067.4 |  | 3,385.6 |  | 7,681.8 |
Total property, plant and equipment | $ | 11,185.1 |  | $ | 3,488.9 |  | $ | 7,696.2 |
(A) | This amount includes a plant acquisition adjustment of $148.3 million and accumulated amortization of $45.3 million.
(B) | This amount includes a plant acquisition adjustment of $3.3 million and accumulated amortization of $0.6 million.
Schedule of Unamortized Computer Software Costs [Table Text Block] | The following table summarizes the Company's unamortized computer software costs included in intangible plant above.
December 31 (In millions) | 2017 | 2016
OGE Energy (holding company) | $ | — |  | $ | 1.0 |
OG&E | 37.5 |  | 36.5 |
Total | $ | 37.5 |  | $ | 37.5 |
Schedule of Computer Software Costs, Amortization [Table Text Block] | The following table summarizes the Company's amortization expense for computer software costs.
Year Ended December 31 (In millions) | 2017 | 2016 | 2015
OGE Energy (holding company) | $ | 0.2 |  | $ | 1.4 |  | $ | 2.0 |
OG&E | 8.8 |  | 8.0 |  | 6.9 |
Total | $ | 9.0 |  | $ | 9.4 |  | $ | 8.9 |
Schedule of Change in Asset Retirement Obligation [Table Text Block] | The following table summarizes changes to the Company's asset retirement obligations during the years ended December 31, 2017 and 2016.
(In millions) | 2017 | 2016
Balance at January 1 | $ | 69.6 |  | $ | 63.3 |
Accretion expense | 3.1 |  | 2.8 |
Revisions in estimated cash flows (A) | 2.4 |  | 3.6 |
Liabilities settled | — |  | (0.1 | )
Balance at December 31 | $ | 75.1 |  | $ | 69.6 |
(A) | Assumptions changed related to the estimated timing of asbestos abatement and estimated cost of ash pond removal at two of OG&E's generating facilities.
Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block] | The following tables summarize changes in the components of accumulated other comprehensive loss attributable to OGE Energy during 2016 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 (loss) | Prior service cost | Total
Balance at December 31, 2015 | $ | (39.2 | ) | $ | 0.1 |  | $ | 2.5 |  | $ | 1.5 |  | $ | (35.1 | )
Other comprehensive income (loss) before reclassifications | (0.7 | ) | — |  | 0.2 |  | — |  | (0.5 | )
Amounts reclassified from accumulated other comprehensive income (loss) | 2.8 |  | — |  | — |  | (1.5 | ) | 1.3 |
Settlement cost | 5.0 |  | — |  | — |  | — |  | 5.0 |
Net current period other comprehensive income (loss) | 7.1 |  | — |  | 0.2 |  | (1.5 | ) | 5.8 |
Balance at December 31, 2016 | (32.1 | ) | 0.1 |  | 2.7 |  | — |  | (29.3 | )
Other comprehensive income (loss) before reclassifications | 0.4 |  | — |  | (0.6 | ) | 6.3 |  | 6.1 |
Amounts reclassified from accumulated other comprehensive income (loss) | 2.5 |  | (0.1 | ) | — |  | (0.6 | ) | 1.8 |
Cumulative effect of change in accounting principle | (5.7 | ) | — |  | (0.1 | ) | 1.3 |  | (4.5 | )
Settlement cost | 2.2 |  | — |  | 0.5 |  | — |  | 2.7 |
Net current period other comprehensive income (loss) | (0.6 | ) | (0.1 | ) | (0.2 | ) | 7.0 |  | 6.1 |
Balance at December 31, 2017 | $ | (32.7 | ) | $ | — |  | $ | 2.5 |  | $ | 7.0 |  | $ | (23.2 | )
Reclassification out of Accumulated Other Comprehensive Income [Table Text Block] | The following table summarizes significant amounts reclassified out of accumulated other comprehensive loss by the respective line items in net income during the years ended December 31, 2017 and 2016.
Details about Accumulated Other Comprehensive Income (Loss) Components | Amount Reclassified from Accumulated Other Comprehensive Income (Loss) | Affected Line Item in the Consolidated Statements of Comprehensive Income
| Year Ended December 31, |
(In millions) | 2017 | 2016 |
Amortization of Pension Plan and Restoration of Retirement Income Plan items: |  |  |
Actuarial losses (A) | $ | (3.9 | ) | $ | (4.5 | ) | Other Operation and Maintenance Expense
Prior service cost | 0.1 |  | — |  | Other Operation and Maintenance Expense
Settlement (A) | (3.6 | ) | (8.2 | ) | Other Operation and Maintenance Expense
| (7.4 | ) | (12.7 | ) | Income Before Taxes
| (2.8 | ) | (4.9 | ) | Income Tax (Benefit) Expense
| $ | (4.6 | ) | $ | (7.8 | ) | Net Income
Amortization of postretirement benefit plan items: |  |  |
Prior service cost | $ | 0.9 |  | $ | 2.5 |  | Other Operation and Maintenance Expense
Settlement (A) | (0.7 | ) | — |  | Other Operation and Maintenance Expense
| 0.2 |  | 2.5 |  | Income Before Taxes
| 0.1 |  | 1.0 |  | Income Tax (Benefit) Expense
| $ | 0.1 |  | $ | 1.5 |  | Net Income
Total reclassifications for the period | $ | (4.5 | ) | $ | (6.3 | ) | Net Income
(A) | These accumulated other comprehensive loss components are included in the computation of net periodic benefit cost (see Note 11 for additional information).
Schedule of Amounts in Accumulated Other Comprehensive Income (Loss) to be Recognized over Next Fiscal Year [Table Text Block] | The amounts in accumulated other comprehensive loss (gain) at December 31, 2017 that are expected to be recognized into earnings in 2018 are as follows:
(In millions) |
Pension Plan and Restoration of Retirement Income Plan: |
Net loss | $ | (3.7 | )
Prior service cost | (0.1 | )
Postretirement Benefit Plans: |
Prior service cost | 2.3 |
Total, net of tax | $ | (1.5 | )

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- Definition Tabular disclosure of physical assets used in the normal conduct of business and not intended for resale. Includes, but is not limited to, balances by class of assets, depreciation and depletion expense and method used, including composite depreciation, and accumulated deprecation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(13)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Tabular disclosure of proportionate interests in jointly owned utility plants. Includes the amount of plants in service, the accumulated provision for depreciation (if available), the amount of plants under construction, and the proportionate share. The amounts presented may be further subdivided to show amounts applicable to plant subcategories (for example, production, transmission, distribution). Includes statements that the amounts represent the utility's share in each joint plant and that it must provide its own financing. Information concerning two or more generating plants on the same site may be combined. States that the utility's share of direct expenses of the joint plants is included in the corresponding operating expenses on its income statement (for example, fuel, maintenance of plant, other operating expense). If the share of direct expenses is charged to purchased power, then disclose the amount so charged and the proportionate amounts charged to specific operating expenses on the records maintained for the joint plants. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 360 -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 10.C) -URI http://asc.fasb.org/extlink&oid=6500719&loc=d3e659983-123033 + Details
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