Derivative Instruments, Hedging Activities and Fair Value Measurements, Asset Balance Sheet Offsetting (Details) (Commodity derivatives [Member], USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Commodity derivatives [Member]
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets	$ 69.5	$ 61.6
Gross Amounts Offset in the Balance Sheet	0	0
Amounts of Assets Presented in the Balance Sheet	69.5	61.6
Financial Instruments	(44.9)	(38.7)
Cash Collateral Received	0	(15.2)
Cash Collateral Paid	(16.1)	0
Amounts That Would Have Been Presented On Net Basis	$ 8.5	$ 7.7