Change in discounted future net cash flows (Details 8) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Beginning of year	$ 978,800	$ 896,100	$ 658,800
Net revenues from production	(280,800)	(301,500)	(270,000)
Net change in sales prices and production costs related to future production	(406,300)	82,300	362,400
Extensions and discoveries, net of future production-related costs	355,300	226,300	130,500
Improved recovery, net of future production-related costs	0	0	0
Purchases of proved reserves, net of future production-related costs	0	9,500	99,800
Sales of proved reserves	(2,600)	0	(500)
Changes in estimated future development costs	37,600	51,100	34,100
Development costs incurred during the current year	77,700	56,300	43,100
Accretion of discount	121,400	105,000	76,500
Net change in income taxes	110,000	(55,800)	(103,300)
Revisions of previous estimates	(100,700)	(92,900)	(132,000)
Other	(7,000)	2,400	(3,300)
Net change	(95,400)	82,700	237,300
End of year	$ 883,400	$ 978,800	$ 896,100