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Risk Management Activities (Narrative) (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Apr. 03, 2017
Dec. 31, 2016
Derivative [Line Items]        
Notional Amount of Forward-Starting Interest rate Swap Agreements       $ 1,500.0
Risk management assets        
Derivative [Line Items]        
Gain (Loss) on Cash Flow Hedge Ineffectiveness, Net $ 0.0 $ 0.0    
Subsequent Event        
Derivative [Line Items]        
Notional Amount of Forward-Starting Interest rate Swap Agreements     $ 200.0