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Fair Value Measurements (Tables) | 9 Months Ended
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Sep. 30, 2019
Fair Value Disclosures [Abstract] |
Schedule of Fair Value and Carrying Amount of PRM Financial Instruments [Table Text Block] | The following table summarizes the carrying amount and fair value of the Company's financial instruments at September 30, 2019 and December 31, 2018, as well as the classification level within the fair value hierarchy.
| September 30, | December 31, |
| 2019 | 2018 |
(In millions) | Carrying Amount | Fair Value | Carrying Amount | Fair Value | Classification
Long-term Debt (including Long-term Debt due within one year): |  |  |  |  |
Senior Notes | $ | 3,049.7 |  | $ | 3,580.0 |  | $ | 3,001.9 |  | $ | 3,178.2 |  | Level 2
OG&E Industrial Authority Bonds | $ | 135.4 |  | $ | 135.4 |  | $ | 135.4 |  | $ | 135.4 |  | Level 2
Tinker Debt | $ | 9.6 |  | $ | 10.3 |  | $ | 9.6 |  | $ | 8.7 |  | Level 3

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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=118260190&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=117815213&loc=d3e19207-110258 + Details
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