Investment in Unconsolidated Affiliates Reconciliation of Equity in Earnings of Unconsolidated Affiliates (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Reconciliation of Equity in Earnings of Unconsolidated Affiliates [Line Items]
Net income		$ 123.0	$ 129.0	$ 351.0	$ 320.0
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity		$ 659.8		$ 659.8		$ 680.3
Limited Liability Company or Limited Partnership, Members or Limited Partners, Ownership Interest		25.50%	25.60%	25.50%	25.60%
Proportionate Unconsolidated Affiliate Net Income		$ 31.2	$ 33.0	$ 89.4	$ 82.0
Amortization of basis difference	[1]	7.1	7.1	15.4	21.3
Equity in earnings of unconsolidated affiliates		38.3	40.1	104.8	103.3
Timing Differences Related to Equity Method Investee Net Income		0.0	0.0	0.1	0.0
Net Income Used to Calculate Equity in Earnings		$ 123.0	$ 129.0	$ 351.1	$ 320.0
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]
Amortization of basis difference	[2]			$ 20.5
Enable Midstream Partners [Member]
Reconciliation of Equity in Earnings of Unconsolidated Affiliates [Line Items]
Share Price		$ 12.03		$ 12.03

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

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