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Investment in Unconsolidated Affiliate and Related Party Transactions (Details) - USD ($)
$ / shares in Units, shares in Millions, $ in Millions
 1 Months Ended 3 Months Ended 9 Months Ended 12 Months Ended
Apr. 30, 2014
Sep. 30, 2015
Sep. 30, 2015
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
May 01, 2013
Schedule of Equity Method Investments [Line Items]              
Accounts receivable - unconsolidated affiliates       $ 1.9 $ 2.5    
Limited Partner Units Owned       111.0      
Distribution Made to Limited Partner, Distributions Declared, Per Unit       $ 0.31800      
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity       $ 714.2 743.7    
Equity in earnings of unconsolidated affiliates       131.2 101.8 $ 15.5  
Enable Midstream Partners [Member]              
Schedule of Equity Method Investments [Line Items]              
Goodwill, Impairment Loss     $ 1,086.4        
Partners' Capital Account, Units, Sold in Public Offering 25.0            
Distributions received       $ 141.2 $ 141.2 $ 139.3  
Enogex LLC [Member]              
Schedule of Equity Method Investments [Line Items]              
Percentage of Enogex LLC Contributed             100.00%
Increase in fair value of net assets             $ 2,200.0
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%  
Subordinated Units Held by Limited Partners of the LLC or LP       68.2      
OGE Energy [Member]              
Schedule of Equity Method Investments [Line Items]              
Goodwill, Impairment Loss   $ 108.4   $ 0.0 $ (2.6) $ (178.4)  
Percentage Share of Management Rights       50.00%      
CenterPoint [Member]              
Schedule of Equity Method Investments [Line Items]              
Percentage Share of Management Rights       50.00%      
Natural Gas Midstream Operations [Member]              
Schedule of Equity Method Investments [Line Items]              
Equity in earnings of unconsolidated affiliates       $ 131.2 $ 101.8 $ 15.5