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Investment in Unconsolidated Affiliate and Related Party Transactions Reconciliation of Basis Difference (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Reconciliation of Basis Difference [Line Items]      
Equity in Earnings Amortization of Basis Difference $ 11.2 $ 11.3 $ 11.6
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity 680.3 714.2  
Equity in Earnings Change in Basis Difference (5.5)    
Elimination of Enable fair value step up 17.2 $ 17.2 $ 16.9
OGE Energy [Member]      
Reconciliation of Basis Difference [Line Items]      
Equity in Earnings Amortization of Basis Difference $ 11.2