Derivative Instruments, Hedging Activities and Fair Value Measurements, Liability Balance Sheet Offsetting (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Interest rate derivatives [Member]
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized Liabilities	$ 1.7	$ 1.1
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Amounts of Liabilities Presented in the Balance Sheet	1.7	1.1
Financial Instruments	(0.1)	(0.2)
Cash Collateral Paid	0.0	0.0
Amounts That Would Have Been Presented On Net Basis	1.6	0.9
Commodity Derivatives [Member]
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized Liabilities	176.9	739.4
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Amounts of Liabilities Presented in the Balance Sheet	176.9	739.4
Financial Instruments	(157.8)	(526.8)
Cash Collateral Paid	(17.3)	(212.4)
Amounts That Would Have Been Presented On Net Basis	$ 1.8	$ 0.2