Investment in Unconsolidated Affiliate Reconciliation of Equity in Earnings of Unconsolidated Affiliates (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020		Dec. 31, 2019	Dec. 31, 2018
Schedule of Equity Method Investments [Line Items]
Limited Liability Company or Limited Partnership, Members or Limited Partners, Ownership Interest		25.50%		25.50%	25.60%
Proportionate Unconsolidated Affiliate Net Income		$ 13.2		$ 91.8	$ 124.4
Amortization of basis difference and dilution recognition	[1]	98.8		22.1	28.4
Equity Method Investment, Other than Temporary Impairment		(780.0)		0.0	0.0
Equity in earnings of unconsolidated affiliates	[2]	(668.0)	[3]	113.9	152.8
Net income (loss)		(173.7)		433.6	425.5
Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member]
Schedule of Equity Method Investments [Line Items]
Net income (loss)		52.0		360.0	485.3
Natural Gas Midstream Operations [Member]
Schedule of Equity Method Investments [Line Items]
Equity in earnings of unconsolidated affiliates		(668.0)	[3]	113.9	152.8
Net income (loss)		$ (515.0)		$ 81.4	$ 108.8
OGE Holdings [Member]
Schedule of Equity Method Investments [Line Items]
Limited Liability Company or Limited Partnership, Members or Limited Partners, Ownership Interest		25.50%
OGE Energy [Member]
Schedule of Equity Method Investments [Line Items]
Amortization of basis difference and dilution recognition	[4]	$ 100.2

[1] Includes loss on dilution, net of proportional basis difference recognition.

[2] For the year ended December 31, 2020, Enable recorded a $225.0 million impairment on its SESH equity method investment. Enable estimated the fair value of this equity method investment was below the carrying value at September 30, 2020 and concluded the decline in value was other than temporary due to the expiration of a transportation contract and the current status of renewal negotiations. The impairment ran through OGE Energy's portion of Enable net income and was offset by basis differences that flow through the amortization of basis difference and dilution recognition line item above.

[3] In March 2020, OGE Energy recorded a $780.0 million impairment on its investment in Enable, as further discussed in Notes 5 and 7.

[4] Includes proportional basis difference recognition due to dilution.