Derivative Instruments, Hedging Activities and Fair Value Measurements, Liability Balance Sheet Offsetting (Details) (Commodity derivatives [Member], USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Commodity derivatives [Member]
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized Liabilities	$ 51.9	$ 45.5
Gross Amounts Offset in the Balance Sheet	0	0
Amounts of Liabilities Presented in the Balance Sheet	51.9	45.5
Financial Instruments	(44.9)	(38.7)
Cash Collateral Paid	0	(4.3)
Amounts That Would Have Been Presented On Net Basis	$ 7.0	$ 2.5