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Fair Value Measurements (Tables) | 12 Months Ended
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Dec. 31, 2018
Fair Value Disclosures [Abstract] |
Fair Value, by Balance Sheet Grouping [Table Text Block] | The following table summarizes the fair value and carrying amount of the Company's financial instruments at December 31, 2018 and 2017.
| 2018 | 2017
December 31 (In millions) | Carrying Amount | Fair Value | Carrying Amount | Fair Value
Long-term Debt (including Long-term Debt due within one year): |  |  |  |
Senior Notes | $ | 3,001.9 |  | $ | 3,178.2 |  | $ | 2,854.3 |  | $ | 3,242.8 |
OG&E Industrial Authority Bonds | $ | 135.4 |  | $ | 135.4 |  | $ | 135.4 |  | $ | 135.4 |
Tinker Debt | $ | 9.6 |  | $ | 8.7 |  | $ | 9.7 |  | $ | 9.8 |

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- Definition Tabular disclosure of the fair value of financial instruments, including financial assets and financial liabilities, and the measurements of those instruments, assets, and liabilities. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=109250915&loc=d3e13467-108611 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258 + Details
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