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Investment in Unconsolidated Affiliate and Related Party Transactions Reconciliation of Equity in Earnings of Unconsolidated Affiliates (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Sep. 30, 2015
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Schedule of Equity Method Investments [Line Items]        
Net income (loss)   $ 400.3 $ 289.5 $ (752.0)
Timing Differences Related to Equity Method Investee Net Income   0.0 (12.2) 12.1
Net Income Used to Calculate Equity in Earnings   400.3 268.2 (739.9)
Proportionate Unconsolidated Affiliate Net Income   102.7 70.7 (194.4)
OGE Energy's share of Enable net income (loss)   102.7 73.3 (16.0)
Amortization of basis difference   11.3 11.6 13.5
Elimination of Enable fair value step up   17.2 16.9 18.0
Equity in earnings of unconsolidated affiliates   131.2 101.8 15.5
Natural Gas Midstream Operations [Member]        
Schedule of Equity Method Investments [Line Items]        
Equity in earnings of unconsolidated affiliates   131.2 101.8 15.5
Preferred Partner [Member]        
Schedule of Equity Method Investments [Line Items]        
Distribution Made to Limited Partner, Cash Distributions Paid   $ 0.0 $ (9.1) $ 0.0
OGE Holdings [Member]        
Schedule of Equity Method Investments [Line Items]        
Limited Liability Company or Limited Partnership, Members or Limited Partners, Ownership Interest   25.70% 25.70% 26.30%
OGE Energy [Member]        
Schedule of Equity Method Investments [Line Items]        
Goodwill, Impairment Loss $ (108.4) $ 0.0 $ 2.6 $ 178.4