Investment in Unconsolidated Affiliates Reconciliation of Equity in Earnings of Unconsolidated Affiliates (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Reconciliation of Equity in Earnings of Unconsolidated Affiliates [Line Items]
Net income	$ 129.0	$ 104.0	$ 320.0	$ 301.0
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity	689.7		689.7		$ 714.2
Proportionate Unconsolidated Affiliate Net Income	33.0	26.5	82.0	77.2
Amortization of basis difference	2.8	2.8	8.4	8.5
Elimination of Enable fair value step up	(4.3)	(4.3)	(12.9)	(12.9)
Equity in earnings of unconsolidated affiliates	$ 40.1	$ 33.6	103.3	$ 98.6
Equity in Earnings Change in Basis Difference			$ (3.2)
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%
OGE Energy [Member]
Reconciliation of Equity in Earnings of Unconsolidated Affiliates [Line Items]
Amortization of basis difference			$ 8.4