Derivative Contracts Schedule of Derivatives Financial Statement Presentation (Details) - Fair Value, Inputs, Level 2 [Member] - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Derivative [Line Items]
Derivative Asset, Fair Value, Gross	$ 17.9	$ 123.6
Derivative Liability, Fair Value, Gross	18.5	1.1
Short-term [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross	2.7	88.2
Derivative Liability, Fair Value, Gross	17.6	0.4
Long-term [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross	15.2	35.4
Derivative Liability, Fair Value, Gross	0.9	0.7
Fair Value, Measurements, Recurring [Member] | Commodity Contract [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross	17.9	123.6
Derivative Liability, Fair Value, Gross	18.5	1.1
Fair Value, Measurements, Recurring [Member] | Commodity Contract [Member] | Short-term [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross	2.7	88.2
Derivative Liability, Fair Value, Gross	17.6	0.4
Fair Value, Measurements, Recurring [Member] | Commodity Contract [Member] | Long-term [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross	15.2	35.4
Derivative Liability, Fair Value, Gross	$ 0.9	$ 0.7