Derivative Contracts Schedule of Derivatives Financial Statement Presentation (Details) - Fair Value, Inputs, Level 2 [Member] - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 155.4	$ 171.0
Derivative Liability, Fair Value, Gross	3.6	5.8
Commodity Contract [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	155.4	171.0
Derivative Liability, Fair Value, Gross	3.6	5.8
Commodity Contract [Member] | Short-term [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	134.5	147.8
Derivative Liability, Fair Value, Gross	0.1	1.8
Commodity Contract [Member] | Long-term [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	20.9	23.2
Derivative Liability, Fair Value, Gross	$ 3.5	$ 4.0