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Derivative Instruments and Hedging Activities, Fair Values Derivatives, Balance Sheet Location, by Derivative Contract Type (Details) - USD ($)
$ in Millions
Jun. 30, 2017
Dec. 31, 2016
Derivatives, Fair Value [Line Items]    
Derivative assets, Current $ 35.7 $ 16.8
Derivative assets, Long-term 17.3 5.1
Derivative assets 53.0 21.9
Derivative liabilities 15.0 75.2
Derivative liabilities, Current 9.5 49.1
Derivative liabilities, Long-term 5.5 26.1
Designated as Hedging Instrument [Member]    
Derivatives, Fair Value [Line Items]    
Derivative assets 52.7 21.8
Derivative liabilities 14.1 74.7
Designated as Hedging Instrument [Member] | Commodity Contracts [Member] | Current Liabilities from Risk Management Activities [Member]    
Derivatives, Fair Value [Line Items]    
Derivative liabilities 8.6 48.6
Designated as Hedging Instrument [Member] | Commodity Contracts [Member] | Long-Term Liabilities from Risk Management Activities [Member]    
Derivatives, Fair Value [Line Items]    
Derivative liabilities 5.5 26.1
Designated as Hedging Instrument [Member] | Commodity Contracts [Member] | Current Assets from Risk Management Activities [Member]    
Derivatives, Fair Value [Line Items]    
Derivative assets 35.4 16.7
Designated as Hedging Instrument [Member] | Commodity Contracts [Member] | Long-Term Assets from Risk Management Activities [Member]    
Derivatives, Fair Value [Line Items]    
Derivative assets 17.3 5.1
Not Designated as Hedging Instrument [Member]    
Derivatives, Fair Value [Line Items]    
Derivative liabilities 0.9 0.5
Not Designated as Hedging Instrument [Member] | Commodity Contracts [Member] | Current Liabilities from Risk Management Activities [Member]    
Derivatives, Fair Value [Line Items]    
Derivative liabilities 0.9 0.5
Not Designated as Hedging Instrument [Member] | Commodity Contracts [Member] | Current Assets from Risk Management Activities [Member]    
Derivatives, Fair Value [Line Items]    
Derivative assets 0.3 0.1
Not Designated as Hedging Instrument [Member] | Commodity Contracts [Member] | Long-Term Assets from Risk Management Activities [Member]    
Derivatives, Fair Value [Line Items]    
Derivative assets $ 0.3 $ 0.1