Change in discounted future net cash flows (Details 7) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Change in the standardized measure of discounted future net cash flows [Roll Forward]
Beginning of year	$ 1,014,000	$ 883,400	$ 978,800
Net revenues from production	(368,900)	(398,000)	(280,800)
Net change in sales prices and production costs related to future production	86,300	162,200	(406,300)
Extensions and discoveries, net of future production-related costs	231,900	366,500	355,300
Improved recovery, net of future production-related costs	0	0	0
Purchases of proved reserves, net of future production-related costs	103,800	0	0
Sales of proved reserves	(219,300)	(37,800)	(2,600)
Changes in estimated future development costs	65,100	6,700	37,600
Development costs incurred during the current year	104,600	141,500	77,700
Accretion of discount	109,400	94,600	121,400
Net change in income taxes	(33,400)	(141,400)	110,000
Revisions of previous estimates	(16,300)	(55,800)	(100,700)
Other	(2,300)	(7,900)	(7,000)
Net change	60,900	130,600	(95,400)
End of year	$ 1,074,900	$ 1,014,000	$ 883,400