Estimated discounted future cash flows of total proved reserves (Details 6) (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]
Future cash inflows	$ 4,507,000	$ 3,696,200	$ 4,188,000
Future production costs	1,734,800	1,536,500	1,560,300
Future development costs	403,000	301,600	285,300
Future net cash flows before income taxes	2,369,200	1,858,100	2,342,400
Future income tax expense	545,200	304,900	531,100
Future net cash flows	1,824,000	1,553,200	1,811,300
10% annual discount for estimated timing of cash flows	810,000	669,800	832,500
Discounted future net cash flows relating to proved oil, NGL and natural gas reserves	$ 1,014,000	$ 883,400	$ 978,800