Derivative Instruments, Hedging Activities and Fair Value Measurements, Derivative Fair Value Amounts (Details) (Commodity derivatives [Member], USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Derivatives, Fair Value [Line Items]
Asset Derivatives	$ 132.7	$ 226.6
Liability Derivatives	108.0	147.4
Derivatives designated as hedging instruments [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	124.6	217.9
Liability Derivatives	101.2	145.3
Derivatives designated as hedging instruments [Member] | Other current assets [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	123.9	217.9
Derivatives designated as hedging instruments [Member] | Other assets [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	0.7	0
Derivatives designated as hedging instruments [Member] | Other current liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives	100.3	145.3
Derivatives designated as hedging instruments [Member] | Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives	0.9	0
Derivatives not designated as hedging instruments [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	8.1	8.7
Liability Derivatives	6.8	2.1
Derivatives not designated as hedging instruments [Member] | Other current assets [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	7.8	8.1
Derivatives not designated as hedging instruments [Member] | Other assets [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	0.3	0.6
Derivatives not designated as hedging instruments [Member] | Other current liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives	5.5	0.7
Derivatives not designated as hedging instruments [Member] | Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives	$ 1.3	$ 1.4