Derivative Contracts Schedule of Derivatives Financial Statement Presentation (Details) - Fair Value, Inputs, Level 2 [Member] - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Derivative [Line Items]
Derivative Asset, Fair Value, Gross	$ 93.0	$ 123.6
Derivative Liability, Fair Value, Gross	1.2	1.1
Short-term [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross	69.8	88.2
Derivative Liability, Fair Value, Gross	0.8	0.4
Long-term [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross	23.2	35.4
Derivative Liability, Fair Value, Gross	0.4	0.7
Fair Value, Measurements, Recurring [Member] | Commodity Contract [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross	93.0	123.6
Derivative Liability, Fair Value, Gross	1.2	1.1
Fair Value, Measurements, Recurring [Member] | Commodity Contract [Member] | Short-term [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross	69.8	88.2
Derivative Liability, Fair Value, Gross	0.8	0.4
Fair Value, Measurements, Recurring [Member] | Commodity Contract [Member] | Long-term [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross	23.2	35.4
Derivative Liability, Fair Value, Gross	$ 0.4	$ 0.7