Derivative Instruments, Hedging Activities and Fair Value Measurements, Asset Balance Sheet Offsetting (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Interest rate derivatives [Member]
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets	$ 6.8	$ 3.2
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Amounts of Assets Presented in the Balance Sheet	6.8	3.2
Financial Instruments	0.0	(3.2)
Cash Collateral Received	0.0	0.0
Cash Collateral Paid	0.0	0.0
Amounts That Would Have Been Presented On Net Basis	6.8	0.0
Commodity derivatives [Member]
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets	231.0	255.6
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Amounts of Assets Presented in the Balance Sheet	231.0	255.6
Financial Instruments	(228.9)	(143.0)
Cash Collateral Received	0.0	(40.1)
Cash Collateral Paid	0.0	(72.2)
Amounts That Would Have Been Presented On Net Basis	$ 2.1	$ 0.3