Supplemental Oil and Gas Reserve Information (Details 2) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Increase (Decrease) in Standardized Measure of Discounted Future Net Cash Flow Relating to Proved Oil and Gas Reserves [Roll Forward]
Standardized measure, beginning of period	$ 938,512	$ 684,438	$ 364,273
Net changes in sales prices, net of production costs	(196,930)	206,357	192,193
Revisions of quantity estimates	(144,899)	(53,089)	56,190
Accretion of discount	137,369	99,028	45,963
Changes in future development costs, including development costs incurred that reduced future development costs	148,733	84,638	39,689
Changes in timing and other	(58,322)	(45,055)	20,839
Net change in income taxes	76,593	(130,562)	(210,090)
Future abandonment cost, net of salvage	(9,230)	925	(1,107)
Extensions and discoveries	289,999	399,068	441,719
Sales, net of production costs	(277,248)	(305,769)	(244,792)
Purchases of minerals-in-place	80,744	0	9,290
Sales of minerals-in-place	(45,490)	(1,466)	(29,729)
Standardized measure, end of period	$ 939,831	$ 938,512	$ 684,438