Investment in Unconsolidated Affiliates Reconciliation of Equity in Earnings of Unconsolidated Affiliates (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Reconciliation of Equity in Earnings of Unconsolidated Affiliates [Line Items]
Net income	$ 113.2	$ 104.6
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity	$ 670.4		$ 680.3
Limited Liability Company or Limited Partnership, Members or Limited Partners, Ownership Interest	25.50%	25.60%
Proportionate Unconsolidated Affiliate Net Income	$ 28.9	$ 26.8
Amortization of basis difference	1.8	7.1
Elimination of Enable fair value step up	0.0	0.0
Proportional Basis Difference Recognition	0.0
Dilution and Impairment Loss	0.0	0.0
Equity in earnings of unconsolidated affiliates	30.7	33.9
Timing Differences Related to Equity Method Investee Net Income	0.1	0.0
Net Income Used to Calculate Equity in Earnings	$ 113.3	$ 104.6
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	$ 9.9