Rate Matters and Regulation (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Public Utilities, Approved Equity Capital Structure, Percentage	53.00%
Public Utilities, Approved Debt Capital Structure, Percentage	47.00%
Interim Rate Refund Amount	$ 47.5
Regulatory Assets, Current	40.8	$ 121.8
Recommended Disallowance for Fuel Adjustment	$ 3.3
Administrative Law Judge [Member]
Recommended Return on Equity	9.87%
Recommended Increase (Decrease) in Revenue	$ 40.7
Recommended Capital Structure, Equity Percentage	53.00%
Recommended Capital Structure, Debt Percentage	47.00%
Oklahoma Corporation Commission [Member]
OG&E's Jurisdictional Revenues	85.00%
Arkansas Public Service Commission [Member]
OG&E's Jurisdictional Revenues	8.00%
Federal Energy Regulatory Commission [Member]
OG&E's Jurisdictional Revenues	7.00%
Dry Scrubber Project [Member]
Estimated Environmental Capital Costs	$ 542.4
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	401.3
Mustang Modernization [Member]
Estimated Environmental Capital Costs	390.0
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	$ 348.4
OKLAHOMA
Public Utilities, Requested Equity Capital Structure, Percentage	53.00%
Public Utilities, Requested Rate Increase (Decrease), Amount	$ 92.5
Public Utilities, Requested Return on Equity, Percentage	10.25%
Investments Since Last Rate Case	$ 1,600.0
Public Utilities, Interim Rate Increase (Decrease), Amount	69.5
Interim Rate Revenue Reserved	5.6
Public Utilities, Approved Rate Increase (Decrease), Amount	$ 8.8
Public Utilities, Approved Return on Equity, Percentage	9.50%
Approved Depreciation Reduction from ALJ Recommendation	$ 28.6
ALJ Depreciation Recommendation	$ 36.4
Approved Recovery of Short-term Incentive Comp	50.00%
ARKANSAS
Public Utilities, Requested Rate Increase (Decrease), Amount	$ 16.5
Public Utilities, Requested Return on Equity, Percentage	10.25%
Investments Since Last Rate Case	$ 3,000.0
Settlement of Rate Increase	$ 7.1
Settlement of Return on Equity, Percentage	9.50%
Settlement of Equity Capital Structure, Percentage	50.00%
Oklahoma demand program rider under recovery [Member]
Regulatory Assets, Current	$ 31.6	$ 51.0
January 16, 2018 [Member] \| OKLAHOMA
Public Utilities, Requested Rate Increase (Decrease), Amount	$ 1.9
Public Utilities, Requested Return on Equity, Percentage	9.90%
FERC [Member]
Recommended Common Equity Percentage	7.85%
Public Utilities, Approved Return on Equity, Percentage	10.60%
Revenue impact of recommended change in return on common equity	$ 1.5

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Interim Rate Refund Amount

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Recommended Capital Structure, Debt Percentage

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Recommended Disallowance for Fuel Adjustment

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Percentage of public utility's approved debt to capital, including debt and equity.

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Amount of public utility's approved rate increase (decrease) by regulatory agency.

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Amount of public utility's interim rate increase (decrease) allowed while waiting final decision of regulatory agency.

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Percentage of public utility's requested equity to capital, including debt and equity.

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Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are expected to be recovered through revenue sources within one year or the normal operating cycle, if longer. Such costs are capitalized if they meet both of the following criteria: a. It is probable that future revenue in an amount at least equal to the capitalized cost will result from inclusion of that cost in allowable costs for rate-making purposes. b. Based on available evidence, the future revenue will be provided to permit recovery of the previously incurred cost rather than to provide for expected levels of similar future costs. If the revenue will be provided through an automatic rate-adjustment clause, this criterion requires that the regulator's intent clearly be to permit recovery of the previously incurred cost.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 340
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382

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