Derivative Instruments, Hedging Activities and Fair Value Measurements, Asset Balance Sheet Offsetting (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Commodity derivatives [Member]
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets	$ 226.6	$ 41.3
Gross Amounts Offset in the Balance Sheet	0	0
Amounts of Assets Presented in the Balance Sheet	226.6	41.3
Financial Instruments	(147.3)	(41.0)
Cash Collateral Received	(23.9)	0
Amounts That Would Have Been Presented On Net Basis	55.4	0.3
Interest rate derivatives [Member]
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets		32.6
Gross Amounts Offset in the Balance Sheet		0
Amounts of Assets Presented in the Balance Sheet		32.6
Financial Instruments		(2.6)
Cash Collateral Received		0
Amounts That Would Have Been Presented On Net Basis		$ 30.0