Business segments (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Segment Reporting Information:
Revenues	$ 224,556	$ 71,815	$ 448,516	$ 257,286
Segment income (loss) before income taxes	89,473	(23,342)	39,143	(56,698)
Total Assets	2,178,509		2,178,509		2,025,203
Oil and gas exploration and production segment
Segment Reporting Information:
Revenues	211,259	54,869	385,867	208,591
Segment income (loss) before income taxes	136,886	5,853	174,612	31,218
Total Assets	1,930,333		1,930,333		1,755,263
Gas marketing segment
Segment Reporting Information:
Revenues	13,297	16,946	62,649	48,695
Segment income (loss) before income taxes	(51)	(181)	3	(233)
Total Assets	9,305		9,305		20,342
Corporate and other segment
Segment Reporting Information:
Segment income (loss) before income taxes	(47,362)	(29,014)	(135,472)	(87,683)
Total Assets	56,802		56,802		63,453
Assets Held-for-sale
Segment Reporting Information:
Total Assets	$ 182,069		$ 182,069		$ 186,145