Fair Value Measurements (Fair Value Hierarchy) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Oil derivative contracts - current assets	$ 187	$ 11,936
Total Assets	187	11,936
Oil derivative contracts - current liabilities	(53,865)	(8,346)
Oil derivative contracts - long-term liabilities	(5,087)	0
Total Liabilities	(58,952)	(8,346)
Quoted Prices in Active Markets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Oil derivative contracts - current assets	0	0
Total Assets	0	0
Oil derivative contracts - current liabilities	0	0
Oil derivative contracts - long-term liabilities	0
Total Liabilities	0	0
Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Oil derivative contracts - current assets	187	8,503
Total Assets	187	8,503
Oil derivative contracts - current liabilities	(53,865)	(6,522)
Oil derivative contracts - long-term liabilities	(5,087)
Total Liabilities	(58,952)	(6,522)
Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Oil derivative contracts - current assets	0	3,433
Total Assets	0	3,433
Oil derivative contracts - current liabilities	0	(1,824)
Oil derivative contracts - long-term liabilities	0
Total Liabilities	$ 0	$ (1,824)

Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

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