Derivative Instruments, Hedging Activities and Fair Value Measurements, Liability Balance Sheet Offsetting (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Interest rate derivatives [Member]
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized Liabilities	$ 5.2	$ 7.8
Gross Amounts Offset in the Balance Sheet	0	0
Amounts of Liabilities Presented in the Balance Sheet	5.2	7.8
Financial Instruments	(1.7)	(2.6)
Cash Collateral Paid	0	0
Amounts That Would Have Been Presented On Net Basis	3.5	5.2
Commodity derivatives [Member]
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized Liabilities	38.3	55.1
Gross Amounts Offset in the Balance Sheet	0	0
Amounts of Liabilities Presented in the Balance Sheet	38.3	55.1
Financial Instruments	(24.8)	(41.0)
Cash Collateral Paid	(4.1)	(9.3)
Amounts That Would Have Been Presented On Net Basis	$ 9.4	$ 4.8