Supplementary Information on Oil and Gas Operations—Unaudited - Changes in Standardized Measure (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
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	$ 451,180	$ 230,823	$ 303,881
Sales and transfers of oil and gas produced, net of production costs	(173,540)	(134,184)	(92,562)
Changes in price, net of future production costs	37,204	55,601	(140,230)
Extensions and discoveries, net of future production and development costs	237,290	70,181	104,066
Changes in estimated future development costs, net of development costs incurred during this period	11,094	(25,389)	77,188
Revisions of quantity estimates	25,591	58,508	(62,159)
Accretion of discount	47,130	23,776	34,137
Net change in income taxes	(32,034)	(13,182)	30,559
Purchase of reserves in place	0	191,964	0
Sale of reserves in place	(7,240)	(411)	(8,186)
Changes in production rates (timing) and other	(48,113)	(6,507)	(15,871)
Net increase (decrease) in standardized measure	97,382	220,357	(73,058)
Standardized measure at end of year	$ 548,562	$ 451,180	$ 230,823