Annual report pursuant to Section 13 and 15(d)

Summary of Significant Accounting Policies (Assets and Liabilities Measured on Recurring Basis) (Details)

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Summary of Significant Accounting Policies (Assets and Liabilities Measured on Recurring Basis) (Details) - USD ($)
$ in Thousands
Dec. 31, 2019
Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative asset commodity $ 636 $ 3,520
Assets 636 3,520
SARs liability 2,638 1,007
SARs liability 852 625
Liabilities 3,490 1,632
Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative asset commodity 636 3,520
Assets 636 3,520
SARs liability 2,638 1,007
SARs liability 852 625
Liabilities $ 3,490 $ 1,632