Business segments (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Segment Reporting Information:
Revenues	$ 90,831	$ 144,632
Segment income (loss) before income taxes	(113,369)	27,785
Total Assets	2,515,397		$ 2,370,543
Oil and gas exploration and production segment
Segment Reporting Information:
Revenues	88,660	141,399
Segment income (loss) before income taxes	(34,033)	60,526
Total Assets	2,482,677		2,294,288
Gas marketing segment
Segment Reporting Information:
Revenues	2,171	3,233
Segment income (loss) before income taxes	(407)	(25)
Total Assets	4,164		4,217
Corporate and other segment
Segment Reporting Information:
Segment income (loss) before income taxes	(78,929)	$ (32,716)
Total Assets	$ 28,556		$ 72,038