Stockholders' equity, Equity Incentive Plan and 401(k) plan - Stock-based compensation award expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Equity and stock-based compensation
Total stock-based compensation, gross	$ 12,760	$ 44,325	$ 43,297
Less amounts capitalized in evaluated oil and natural gas properties	(4,470)	(7,929)	(7,563)
Total stock-based compensation, net	8,290	36,396	35,734
Restricted stock awards
Equity and stock-based compensation
Total stock-based compensation, gross	13,169	25,271	22,223
Stock option awards
Equity and stock-based compensation
Total stock-based compensation, gross	740	3,862	4,762
Performance share awards
Equity and stock-based compensation
Total stock-based compensation, gross	(1,250)	15,192	16,312
Outperformance share awards
Equity and stock-based compensation
Total stock-based compensation, gross	$ 101	$ 0	$ 0