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Risk Management Activities (Narrative) (Details) - USD ($)
$ in Millions
 9 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Jun. 24, 2016
Dec. 31, 2015
Derivative [Line Items]        
Notional Amount of Forward-Starting Interest rate Swap Agreements $ 1,500.0   $ 500.0  
Swap Forecasted Debt Issuance 2018      
Derivative, Notional Amount, Current $ 250.0      
Risk management assets        
Derivative [Line Items]        
Gain (Loss) on Cash Flow Hedge Ineffectiveness, Net 0.0 $ 0.0    
Derivative Liability, Fair Value, Gross Liability 256.8     $ 17.4
Price Risk Cash Flow Hedge Unrealized Gain (Loss) to be Reclassified During Next 12 Months $ 0.0