Derivative Contracts Schedule of Derivatives Financial Statement Presentation (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 326.2	$ 349.2
Derivative Liability, Fair Value, Gross	0.8	0.3
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	326.2	349.2
Derivative Liability, Fair Value, Gross	0.8	0.3
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Commodity Contract [Member] | Short-term [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	312.5	339.3
Derivative Liability, Fair Value, Gross	0.8	0.3
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Commodity Contract [Member] | Long-term [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	13.7	9.9
Derivative Liability, Fair Value, Gross	$ 0	$ 0