Derivative Contracts Schedule of Derivatives Financial Statement Presentation (Details) - Fair Value, Inputs, Level 2 [Member] - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018
Derivative [Line Items]
Derivative Asset, Fair Value, Gross	$ 6.7	$ 123.6
Derivative Liability, Fair Value, Gross	61.8	1.1
Short-term [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross	0.0	88.2
Derivative Liability, Fair Value, Gross	60.3	0.4
Long-term [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross	6.7	35.4
Derivative Liability, Fair Value, Gross	1.5	0.7
Fair Value, Measurements, Recurring [Member] | Commodity Contract [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross		123.6	$ 6.7
Derivative Liability, Fair Value, Gross	61.8	1.1
Fair Value, Measurements, Recurring [Member] | Commodity Contract [Member] | Short-term [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross	0.0	88.2
Derivative Liability, Fair Value, Gross	60.3	0.4
Fair Value, Measurements, Recurring [Member] | Commodity Contract [Member] | Long-term [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross	6.7	35.4
Derivative Liability, Fair Value, Gross	$ 1.5	$ 0.7