Change in discounted future net cash flows (Details 7) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Beginning of year	$ 883,400	$ 978,800	$ 896,100
Net revenues from production	(398,000)	(280,800)	(301,500)
Net change in sales prices and production costs related to future production	162,200	(406,300)	82,300
Extensions and discoveries, net of future production-related costs	366,500	355,300	226,300
Improved recovery, net of future production-related costs	0	0	0
Purchases of proved reserves, net of future production-related costs	0	0	9,500
Sales of proved reserves	(37,800)	(2,600)	0
Changes in estimated future development costs	6,700	37,600	51,100
Development costs incurred during the current year	141,500	77,700	56,300
Accretion of discount	94,600	121,400	105,000
Net change in income taxes	(141,400)	110,000	(55,800)
Revisions of previous estimates	(55,800)	(100,700)	(92,900)
Other	(7,900)	(7,000)	2,400
Net change	130,600	(95,400)	82,700
End of year	$ 1,014,000	$ 883,400	$ 978,800