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Summary of Significant Accounting Policies Property, Plant and Equipment (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Property, Plant and Equipment [Line Items] |  |  |  |  |
Property, Plant and Equipment, Gross | $ 11,908.7 |  | $ 11,185.1 |  |
Accumulated Depreciation | 3,568.8 |  | 3,488.9 |  |
Net property, plant and equipment | 8,339.9 |  | 7,696.2 |  |
Capitalized Computer Software, Gross | 37.5 |  | 37.5 |  |
Capitalized Computer Software, Amortization | $ 9.0 |  | $ 9.4 |  | $ 8.9
McClain Plant [Member] |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |
Jointly Owned Utility Plant, Proportionate Ownership Share | 77.00% | [1] | 77.00% | [2] |
Property, Plant and Equipment, Gross | $ 226.8 | [1] | $ 234.2 | [2] |
Accumulated Depreciation | 71.4 | [1] | 72.3 | [2] |
Net property, plant and equipment | 155.4 | [1] | 161.9 | [2] |
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress | $ 0.4 |  | $ 0.2 |  |
Redbud Plant [Member] |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |
Jointly Owned Utility Plant, Proportionate Ownership Share | 51.00% | [1],[3] | 51.00% | [2],[4] |
Property, Plant and Equipment, Gross | $ 496.6 | [1],[3] | $ 489.0 | [2],[4] |
Accumulated Depreciation | 136.0 | [1],[3] | 121.0 | [2],[4] |
Net property, plant and equipment | 360.6 | [1],[3] | 368.0 | [2],[4] |
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress | 7.8 |  | 1.8 |  |
Amount of Acquisition Adjustments | 148.3 |  | 148.3 |  |
Public Utilities, Property, Plant and Equipment, Amount of Acquisition Adjustments, Related Accumulated Depreciation | 50.8 |  | 45.3 |  |
OGE Energy [Member] |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |
Property, Plant and Equipment, Gross | 6.1 |  | 117.7 |  |
Accumulated Depreciation | 0.0 |  | 103.3 |  |
Net property, plant and equipment | 6.1 |  | 14.4 |  |
Capitalized Computer Software, Gross | 0.0 |  | 1.0 |  |
Capitalized Computer Software, Amortization | 0.2 |  | 1.4 |  | 2.0
OGE Energy [Member] | Total Property Plant and Equipment [Member] |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |
Property, Plant and Equipment, Gross | 6.1 |  | 117.7 |  |
Accumulated Depreciation | 0.0 |  | 103.3 |  |
Net property, plant and equipment | 6.1 |  | 14.4 |  |
OG&E [Member] |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |
Capitalized Computer Software, Gross | 37.5 |  | 36.5 |  |
Capitalized Computer Software, Amortization | 8.8 |  | 8.0 |  | $ 6.9
OG&E [Member] | Total Property Plant and Equipment [Member] |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |
Property, Plant and Equipment, Gross | 11,902.6 |  | 11,067.4 |  |
Accumulated Depreciation | 3,568.8 |  | 3,385.6 |  |
Net property, plant and equipment | 8,333.8 |  | 7,681.8 |  |
OG&E [Member] | Electric Transmission and Distribution [Member] |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |
Property, Plant and Equipment, Gross | 4,057.1 |  | 3,896.2 |  |
Accumulated Depreciation | 1,259.1 |  | 1,221.5 |  |
Net property, plant and equipment | 2,798.0 |  | 2,674.7 |  |
OG&E [Member] | Electric Generation Equipment [Member] |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |
Property, Plant and Equipment, Gross | 4,475.0 | [5] | 4,155.9 | [6] |
Accumulated Depreciation | 1,493.5 | [5] | 1,493.3 | [6] |
Net property, plant and equipment | 2,981.5 | [5] | 2,662.6 | [6] |
Amount of Acquisition Adjustments | 148.3 |  | 148.3 |  |
Public Utilities, Property, Plant and Equipment, Amount of Acquisition Adjustments, Related Accumulated Depreciation | 50.8 |  | 45.3 |  |
OG&E [Member] | Electric Transmission [Member] |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |
Property, Plant and Equipment, Gross | 2,767.7 | [7] | 2,548.8 | [8] |
Accumulated Depreciation | 506.5 | [7] | 481.3 | [8] |
Net property, plant and equipment | 2,261.2 | [7] | 2,067.5 | [8] |
Amount of Acquisition Adjustments | 3.3 |  | 3.3 |  |
Amount of Acquistion Adjustments Related Accumulated Amortization | 0.6 |  | 0.6 |  |
OG&E [Member] | Finite-Lived Intangible Assets, Major Class Name [Member] |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |
Property, Plant and Equipment, Gross | 181.8 |  | 85.0 |  |
Accumulated Depreciation | 135.8 |  | 43.9 |  |
Net property, plant and equipment | 46.0 |  | 41.1 |  |
OG&E [Member] | Property, Plant and Equipment, Other Types [Member] |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |
Property, Plant and Equipment, Gross | 421.0 |  | 381.5 |  |
Accumulated Depreciation | 173.9 |  | 145.6 |  |
Net property, plant and equipment | $ 247.1 |  | $ 235.9 |  |
[1] | Construction work in progress was $0.4 million and $7.8 million for the McClain and Redbud Plants, respectively.
[2] | Construction work in progress was $0.2 million and $1.8 million for the McClain and Redbud Plants, respectively.
[3] | This amount includes a plant acquisition adjustment of $148.3 million and accumulated amortization of $50.8 million.
[4] | This amount includes a plant acquisition adjustment of $148.3 million and accumulated amortization of $45.3 million
[5] | This amount includes a plant acquisition adjustment of $148.3 million and accumulated amortization of $50.8 million.
[6] | This amount includes a plant acquisition adjustment of $148.3 million and accumulated amortization of $45.3 million.
[7] | This amount includes a plant acquisition adjustment of $3.3 million and accumulated amortization of $0.6 million.
[8] | This amount includes a plant acquisition adjustment of $3.3 million and accumulated amortization of $0.6 million

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