Estimated discounted future cash flows of total proved reserves (Details 7) - USD ($) $ in Thousands	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Oil and Gas Exploration and Production Industries Disclosures [Abstract]
Future cash inflows	$ 5,185,500	$ 4,507,000
Future production costs	1,856,900	1,734,800
Future development costs	570,200	403,000
Future net cash flows before income taxes	2,758,400	2,369,200
Future income tax expense	686,100	545,200
Future net cash flows	2,072,300	1,824,000
10% annual discount for estimated timing of cash flows	997,400	810,000
Discounted future net cash flows relating to proved oil, NGL and natural gas reserves	$ 1,074,900	$ 1,014,000	$ 883,400