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Risk Management Activities (Narrative) (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Dec. 31, 2018
Mar. 31, 2018
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
May 22, 2017
Derivative [Line Items]            
Derivative Instruments, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Net     $ 0.0 $ 0.0 $ 0.0  
Interest Rate Swap            
Derivative [Line Items]            
Derivative, Notional Amount $ 500.0   500.0     $ 950.0
Interest Rate Swap Settled            
Derivative [Line Items]            
Derivative, Notional Amount 750.0   750.0      
Derivative, Gain (Loss) on Derivative, Net [1] $ 25.0 $ 21.2 $ 46.2      
[1] See Note 9, "Risk Management Activities" for additional information.