Supplemental Information on Oil and Gas Producing Activities (Standardized Measure) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Company's Standardized Measures
Future cash inflows	$ 377,144	$ 262,889	$ 192,417
Future production costs	(145,478)	(110,858)	(92,868)
Future development costs	(40,328)	(8,161)	(6,502)
Future income taxes	(30,326)	(13,532)
Future net cash flows	161,012	130,338	93,047
10% annual discount	(70,900)	(55,034)	(34,822)
Standardized Measure	90,112	75,304	58,225
Principal changes in the standardized measures
Standardized measure, beginning balance	75,304	58,225	120,677
Sales of oil and gas produced, net of production costs	(16,880)	(14,744)	(24,586)
Extensions and discoveries	4,392	1,776	343
Net change in prices and production costs related to future production	21,661	31,546	(95,294)
Development costs incurred during the year		381	4,231
Changes in estimated future development costs	(18,854)	(2,646)	23,945
Purchases of reserves in place			9,026
Revisions of quantity estimates	19,100	2,936	(47,810)
Accretion of discount	7,818	5,823	14,022
Net change in income taxes	(6,902)	(2,879)	33,512
Changes in timing and other	4,473	(5,114)	20,159
Aggregate change	14,808	17,079	(62,452)
Standardized measure, ending balance	$ 90,112	$ 75,304	$ 58,225