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Derivative Instruments, Hedging Activities and Fair Value Measurements, Liability Balance Sheet Offsetting (Details) - Commodity Derivatives [Member] - USD ($)
$ in Millions
Mar. 31, 2019
Dec. 31, 2018
Offsetting Liabilities [Line Items]    
Gross Amounts of Recognized Liabilities $ 108.4 $ 162.4
Gross Amounts Offset in the Balance Sheet 0.0 0.0
Amounts of Liabilities Presented in the Balance Sheet 108.4 162.4
Financial Instruments (108.1) (158.6)
Cash Collateral Received 0.0 0.0
Cash Collateral Paid 27.3 (2.3)
Amounts That Would Have Been Presented On Net Basis $ 27.6 $ 1.5