Derivative Contracts Schedule of Derivatives Financial Statement Presentation (Details) - Fair Value, Inputs, Level 2 [Member] - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 203.4	$ 349.2
Derivative Liability, Fair Value, Gross	2.5	0.3
Commodity Contract [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	203.4	349.2
Derivative Liability, Fair Value, Gross	2.5	0.3
Commodity Contract [Member] | Short-term [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	186.4	339.3
Derivative Liability, Fair Value, Gross	2.5	0.3
Commodity Contract [Member] | Long-term [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	17.0	9.9
Derivative Liability, Fair Value, Gross	$ 0.0	$ 0.0