Derivative Instruments, Hedging Activities and Fair Value Measurements, Asset Balance Sheet Offsetting (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Interest rate derivatives [Member]
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets	$ 36.5	$ 3.2
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Amounts of Assets Presented in the Balance Sheet	36.5	3.2
Financial Instruments	(0.2)	(3.2)
Cash Collateral Paid	0.0	0.0
Cash Collateral Received	0.0	0.0
Amounts That Would Have Been Presented On Net Basis	36.3	0.0
Commodity Derivatives [Member]
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets	541.4	255.6
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Amounts of Assets Presented in the Balance Sheet	541.4	255.6
Financial Instruments	(526.8)	(143.0)
Cash Collateral Paid	0.0	(40.1)
Cash Collateral Received	0.0	(72.2)
Amounts That Would Have Been Presented On Net Basis	$ 14.6	$ 0.3