Derivative Contracts Schedule of Derivatives Financial Statement Presentation (Details) - Fair Value, Inputs, Level 2 [Member] - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
Derivative Asset, Fair Value, Gross	$ 24.4	$ 22.9
Derivative Liability, Fair Value, Gross	166.4	154.8
Short-term [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross	16.3	20.6
Derivative Liability, Fair Value, Gross	130.0	120.8
Long-term [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross	8.1	2.3
Derivative Liability, Fair Value, Gross	36.4	34.0
Fair Value, Measurements, Recurring [Member] | Commodity Contract [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross	24.4	22.9
Derivative Liability, Fair Value, Gross	166.4	154.8
Fair Value, Measurements, Recurring [Member] | Commodity Contract [Member] | Short-term [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross	16.3	20.6
Derivative Liability, Fair Value, Gross	130.0	120.8
Fair Value, Measurements, Recurring [Member] | Commodity Contract [Member] | Long-term [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross	8.1	2.3
Derivative Liability, Fair Value, Gross	$ 36.4	$ 34.0