Annual report pursuant to Section 13 and 15(d)

Derivatives and Fair Value (Effect of Derivative Instruments on the Condensed Consolidated Statements of Operations) (Details)

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Derivatives and Fair Value (Effect of Derivative Instruments on the Condensed Consolidated Statements of Operations) (Details) - Crude Oil Swaps [Member] - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Derivative Instruments, Gain (Loss) [Line Items]      
Derivative, gain (loss) $ (446) $ 4,264 $ (1,032)
Realized Gain - Contract Settlements [Member]      
Derivative Instruments, Gain (Loss) [Line Items]      
Derivative, gain (loss) 2,439 744 195
Unrealized (Gain) Loss [Member]      
Derivative Instruments, Gain (Loss) [Line Items]      
Derivative, gain (loss) $ (2,885) $ 3,520 $ (1,227)