Derivative Instruments, Hedging Activities and Fair Value Measurements, Asset Balance Sheet Offsetting (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Interest rate derivatives [Member]
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets	$ 0.1	$ 36.5
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Amounts of Assets Presented in the Balance Sheet	0.1	36.5
Financial Instruments	(0.1)	(0.2)
Cash Collateral Paid	0.0	0.0
Cash Collateral Received	0.0	0.0
Amounts That Would Have Been Presented On Net Basis	0.0	36.3
Commodity Derivatives [Member]
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets	161.7	541.4
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Amounts of Assets Presented in the Balance Sheet	161.7	541.4
Financial Instruments	(157.8)	(526.8)
Cash Collateral Paid	0.0	0.0
Cash Collateral Received	0.0	0.0
Amounts That Would Have Been Presented On Net Basis	$ 3.9	$ 14.6