Selected Quarterly Information (Unaudited) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Effect of Fourth Quarter Events [Line Items]
Net (loss) gain									$ (38,511)	$ (9,989)	$ (8,513)
Net (loss) gain per share - basic and diluted									$ (0.63)	$ (0.20)	$ (0.21)
Weighted-average shares outstanding-basic and diluted									61,107	50,593	41,238
Quarterly financial information [Member]
Effect of Fourth Quarter Events [Line Items]
Oil and gas sales
Net (loss) gain	$ (31,800)	$ 781	$ (1,241)	$ (6,251)	$ (181)	$ (1,462)	$ (4,971)	$ (3,375)
Net (loss) gain per share - basic and diluted	$ (0.48)	$ 0.02	$ (0.02)	$ (0.11)	$ (0.003)	$ (0.03)	$ (0.10)	$ (0.07)
Weighted-average shares outstanding-basic and diluted	66,135	61,089	59,346	57,504	55,598	53,382	50,245	47,237