Derivative Instruments, Hedging Activities and Fair Value Measurements, Liability Balance Sheet Offsetting (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Commodity derivatives [Member]
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized Liabilities	$ 147.4	$ 55.1
Gross Amounts Offset in the Balance Sheet	0	0
Amounts of Liabilities Presented in the Balance Sheet	147.4	55.1
Financial Instruments	(147.3)	(41.0)
Cash Collateral Paid	0	(9.3)
Amounts That Would Have Been Presented On Net Basis	0.1	4.8
Interest rate derivatives [Member]
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized Liabilities		7.8
Gross Amounts Offset in the Balance Sheet		0
Amounts of Liabilities Presented in the Balance Sheet		7.8
Financial Instruments		(2.6)
Cash Collateral Paid		0
Amounts That Would Have Been Presented On Net Basis		$ 5.2