Oil and Gas Reserve Information - Unaudited - Changes in standardized measure (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2009
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	$ 232,395	$ 303,881		$ 236,375
Sales and transfers of oil and gas produced, net of production costs	(134,184)	(92,562)	(116,398)
Changes in price, net of future production costs	57,293	(138,842)	(10,219)
Extensions and discoveries, net of future production and development costs	70,181	104,066	178,901
Changes in estimated future development costs, net of development costs incurred during this period	(24,327)	69,499	915
Revisions of quantity estimates	57,468	(56,352)	11,236
Accretion of discount	23,927	34,137	25,565
Net change in income taxes	(14,061)	30,617	(18,215)
Purchase of reserves in place	191,964	0	4,805
Sale of reserves in place	(411)	(8,186)	0
Changes in production rates (timing) and other	(6,363)	(13,863)	(9,084)
Net increase (decrease) in standardized measure	221,487	(71,486)	67,506
Standardized measure at end of year	$ 453,882	$ 232,395	$ 303,881	$ 236,375