Derivative Instruments, Hedging Activities and Fair Value Measurements, Derivative Fair Value Amounts (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Interest rate derivatives [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	$ 27.3	$ 32.6
Liability Derivatives	2.6	7.8
Commodity derivatives [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	44.1	41.3
Liability Derivatives	66.2	55.1
Derivatives designated as hedging instruments [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	63.0	63.5
Liability Derivatives	61.9	46.8
Derivatives designated as hedging instruments [Member] \| Interest rate derivatives [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	27.3	32.6
Liability Derivatives	0	0
Derivatives designated as hedging instruments [Member] \| Interest rate derivatives [Member] \| Other current assets [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	20.4	20.2
Derivatives designated as hedging instruments [Member] \| Interest rate derivatives [Member] \| Other assets [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	6.9	12.4
Derivatives designated as hedging instruments [Member] \| Interest rate derivatives [Member] \| Other current liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives	0	0
Derivatives designated as hedging instruments [Member] \| Interest rate derivatives [Member] \| Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives	0	0
Derivatives designated as hedging instruments [Member] \| Commodity derivatives [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	35.7	30.9
Liability Derivatives	61.9	46.8
Derivatives designated as hedging instruments [Member] \| Commodity derivatives [Member] \| Other current assets [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	35.7	30.9
Derivatives designated as hedging instruments [Member] \| Commodity derivatives [Member] \| Other assets [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	0	0
Derivatives designated as hedging instruments [Member] \| Commodity derivatives [Member] \| Other current liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives	61.9	46.5
Derivatives designated as hedging instruments [Member] \| Commodity derivatives [Member] \| Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives	0	0.3
Derivatives not designated as hedging instruments [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	8.4	10.4
Liability Derivatives	6.9	16.1
Derivatives not designated as hedging instruments [Member] \| Interest rate derivatives [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	0	0
Liability Derivatives	2.6	7.8
Derivatives not designated as hedging instruments [Member] \| Interest rate derivatives [Member] \| Other current assets [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	0	0
Derivatives not designated as hedging instruments [Member] \| Interest rate derivatives [Member] \| Other assets [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	0	0
Derivatives not designated as hedging instruments [Member] \| Interest rate derivatives [Member] \| Other current liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives	2.6	7.8
Derivatives not designated as hedging instruments [Member] \| Interest rate derivatives [Member] \| Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives	0	0
Derivatives not designated as hedging instruments [Member] \| Commodity derivatives [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	8.4	10.4
Liability Derivatives	4.3	8.3
Derivatives not designated as hedging instruments [Member] \| Commodity derivatives [Member] \| Other current assets [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	7.5	7.6
Derivatives not designated as hedging instruments [Member] \| Commodity derivatives [Member] \| Other assets [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	0.9	2.8
Derivatives not designated as hedging instruments [Member] \| Commodity derivatives [Member] \| Other current liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives	4.0	5.5
Derivatives not designated as hedging instruments [Member] \| Commodity derivatives [Member] \| Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives	$ 0.3	$ 2.8