Derivative Contracts Schedule of Derivatives Financial Statement Presentation (Details) - Fair Value, Inputs, Level 2 [Member] - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Derivative [Line Items]
Derivative Asset, Fair Value, Gross	$ 17.8	$ 22.9
Derivative Liability, Fair Value, Gross	257.0	154.8
Short-term [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross	16.6	20.6
Derivative Liability, Fair Value, Gross	203.3	120.8
Long-term [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross	1.2	2.3
Derivative Liability, Fair Value, Gross	53.7	34.0
Fair Value, Measurements, Recurring [Member] | Commodity Contract [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross	17.8	22.9
Derivative Liability, Fair Value, Gross	257.0	154.8
Fair Value, Measurements, Recurring [Member] | Commodity Contract [Member] | Short-term [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross	16.6	20.6
Derivative Liability, Fair Value, Gross	203.3	120.8
Fair Value, Measurements, Recurring [Member] | Commodity Contract [Member] | Long-term [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross	1.2	2.3
Derivative Liability, Fair Value, Gross	$ 53.7	$ 34.0