Oil and Gas Reserve Information - Unaudited - Changes in standardized measure (Details) (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 at beginning of year	$ 303,881	$ 236,375	$ 174,288
Sales and transfers of oil and gas produced, net of production costs	(92,562)	(116,398)	(117,572)
Changes in price, net of future production costs	(138,842)	(10,219)	93,702
Extensions and discoveries, net of future production and development costs	104,066	178,901	42,028
Changes in estimated future development costs, net of development costs incurred during this period	69,499	915	5,803
Revisions of quantity estimates	(56,352)	11,236	46,373
Accretion of discount	34,137	25,565	17,700
Net change in income taxes	30,617	(18,215)	(16,568)
Purchase of reserves in place	0	4,805	1,478
Sale of reserves in place	(8,186)	0	(798)
Changes in production rates (timing) and other	(13,863)	(9,084)	(10,059)
Net increase (decrease) in standardized measure	(71,486)	67,506	62,087
Standardized measure at end of year	$ 232,395	$ 303,881	$ 236,375