Oil and Gas Reserve Information - Unaudited - Standardized measure (Details) (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, Future Net Cash Flows [Abstract]
Future cash flows	$ 1,265,663	$ 748,914	$ 1,080,392
Future production costs	(301,710)	(220,750)	(264,219)
Future development costs	(193,985)	(121,346)	(180,846)
Future income taxes	(40,072)	(10,205)	(86,612)
Future net cash flows	729,896	396,613	548,715
10% annual discount	(276,014)	(164,218)	(244,834)
Standardized measure of discounted future net cash flows	$ 453,882	$ 232,395	$ 303,881