Derivative Contracts Schedule of Derivatives Financial Statement Presentation (Details) - Fair Value, Inputs, Level 2 [Member] - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 9.4	$ 0.0
Derivative Liability, Fair Value, Gross	19.7	201.8
Short-term [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	6.9	0.0
Derivative Liability, Fair Value, Gross	16.5	169.8
Long-term [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	2.5	0.0
Derivative Liability, Fair Value, Gross	3.2	32.0
Fair Value, Measurements, Recurring [Member] | Commodity Contract [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	9.4	0.0
Derivative Liability, Fair Value, Gross	19.7	201.8
Fair Value, Measurements, Recurring [Member] | Commodity Contract [Member] | Short-term [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	6.9	0.0
Derivative Liability, Fair Value, Gross	16.5	169.8
Fair Value, Measurements, Recurring [Member] | Commodity Contract [Member] | Long-term [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	2.5	0.0
Derivative Liability, Fair Value, Gross	$ 3.2	$ 32.0