Fair Value Measurements (Fair Value Hierarchy) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Oil derivative contracts - current assets	$ 93,080	$ 0
Oil derivative contract - long-term assets	4,195	0
Total Assets	97,275
Oil derivative contracts - current liabilities	0	(99,061)
Total Liabilities		(99,061)
Quoted Prices in Active Markets (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Oil derivative contracts - current assets	0
Oil derivative contract - long-term assets	0
Total Assets	0
Oil derivative contracts - current liabilities		0
Total Liabilities		0
Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Oil derivative contracts - current assets	81,621
Oil derivative contract - long-term assets	2,030
Total Assets	83,651
Oil derivative contracts - current liabilities		(99,061)
Total Liabilities		(99,061)
Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Oil derivative contracts - current assets	11,459
Oil derivative contract - long-term assets	2,165
Total Assets	$ 13,624
Oil derivative contracts - current liabilities		0
Total Liabilities		$ 0