Derivative Contracts Schedule of Derivatives Financial Statement Presentation (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Derivative [Line Items]
Derivative instruments - assets	$ 0.0	$ 170.0
Derivative instruments - liabilities	201.8	4.8
Commodity Contract [Member] | Short term [Member]
Derivative [Line Items]
Derivative instruments - assets	0.0	146.8
Derivative instruments - liabilities	169.8	0.8
Commodity Contract [Member] | Long term [Member]
Derivative [Line Items]
Derivative instruments - assets	0.0	23.2
Derivative instruments - liabilities	32.0	4.0
Fair Value, Inputs, Level 2 [Member]
Derivative [Line Items]
Derivative instruments - assets	0.0	171.0
Derivative instruments - liabilities	201.8	5.8
Fair Value, Inputs, Level 2 [Member] | Commodity Contract [Member] | Short term [Member]
Derivative [Line Items]
Derivative instruments - assets	0.0	147.8
Derivative instruments - liabilities	169.8	1.8
Fair Value, Inputs, Level 2 [Member] | Commodity Contract [Member] | Long term [Member]
Derivative [Line Items]
Derivative instruments - assets	0.0	23.2
Derivative instruments - liabilities	$ 32.0	$ 4.0