Derivative Instruments, Hedging Activities and Fair Value Measurements, Asset Balance Sheet Offsetting (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Interest rate derivatives [Member]
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets	$ 13.0	$ 0.1
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Amounts of Assets Presented in the Balance Sheet	13.0	0.1
Financial Instruments	0.0	(0.1)
Cash Collateral Received	0.0	0.0
Cash Collateral Paid	0.0	0.0
Amounts That Would Have Been Presented On Net Basis	13.0	0.0
Commodity Derivatives [Member]
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets	191.6	161.7
Gross Amounts Offset in the Balance Sheet	0.0	0.0
Amounts of Assets Presented in the Balance Sheet	191.6	161.7
Financial Instruments	(185.6)	(157.8)
Cash Collateral Received	0.0	0.0
Cash Collateral Paid	0.0	0.0
Amounts That Would Have Been Presented On Net Basis	$ 6.0	$ 3.9