Investment in Unconsolidated Affiliates Reconciliation of Equity in Earnings of Unconsolidated Affiliates (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Reconciliation of Equity in Earnings of Unconsolidated Affiliates [Line Items]
Net income	$ 86.4	$ 86.2	$ 191.0	$ 197.4
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity	696.9		696.9		$ 714.2
Proportionate Unconsolidated Affiliate Net Income	22.2	22.2	49.0	50.7
Amortization of basis difference	2.8	2.9	5.6	5.7
Elimination of Enable fair value step up	4.3	4.3	8.6	8.6
Equity in earnings of unconsolidated affiliates	$ 29.3	$ 29.4	63.2	$ 65.0
Equity in Earnings Change in Basis Difference			$ (3.1)
OGE Holdings [Member]
Reconciliation of Equity in Earnings of Unconsolidated Affiliates [Line Items]
Limited Liability Company or Limited Partnership, Members or Limited Partners, Ownership Interest	25.60%	25.70%	25.60%	25.70%

Equity in Earnings Amortization of Basis Difference

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Equity in Earnings Change in Basis Difference

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Equity in Earnings Elimination of Fair Value Adjustments

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Proportionate Share of Equity Method Investee Net Income, Before Adjustments

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[Line Items] for Reconciliation of Equity in Earnings of Unconsolidated Affiliates [Table]

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Difference between amount at which an investment accounted for under the equity method of accounting is carried (reported) on the balance sheet and amount of underlying equity in net assets the reporting Entity has in the investee.

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The amount of net income (loss) reported by an equity method investment of the entity.

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Percentage investment held by members or limited partners of limited liability company (LLC) or limited partnership (LP).

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