Investment in Unconsolidated Affiliates Reconciliation of Equity in Earnings of Unconsolidated Affiliates (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2020		Mar. 31, 2019	Dec. 31, 2019
Reconciliation of Equity in Earnings of Unconsolidated Affiliates [Line Items]
Equity Method Investment, Summarized Financial Information, Net Income (Loss)		$ 103.0		$ 113.3
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity		$ 1,424.3			$ 652.5
Limited Liability Company or Limited Partnership, Members or Limited Partners, Ownership Interest		25.50%		25.50%
Proportionate Unconsolidated Affiliate Net Income		$ 26.3		$ 28.9
Equity in Earnings Amortization of Basis Difference	[1]	7.2		1.8
Income (Loss) from Equity Method Investments, Total		(746.5)	[2]	30.7
Asset Impairment Charges		$ (780.0)		$ 0.0
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		25.50%
OGE Energy [Member]
Reconciliation of Equity in Earnings of Unconsolidated Affiliates [Line Items]
Equity in Earnings Amortization of Basis Difference	[3]	$ 8.2

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

[2] At March 31, 2020, the Company recorded a $780.0 million impairment on its investment in Enable, as further discussed in Notes 4 and 5.

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