Estimated discounted future cash flows of total proved reserves (Details 7) (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]
Future cash inflows	$ 3,696,200	$ 4,188,000	$ 3,790,700
Future production costs	1,536,500	1,560,300	1,393,000
Future development costs	301,600	285,300	312,500
Future net cash flows before income taxes	1,858,100	2,342,400	2,085,200
Future income tax expense	304,900	531,100	432,800
Future net cash flows	1,553,200	1,811,300	1,652,400
10% annual discount for estimated timing of cash flows	669,800	832,500	756,300
Discounted future net cash flows relating to proved oil, NGL and natural gas reserves	$ 883,400	$ 978,800	$ 896,100