Disclosures about Oil and Natural Gas Activities (Unaudited)--Changes in Standardized Measure of Discounted Future Net Cash Flows (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Principal Sources of Change in Standardized Measure of Discounted Future Net Cash Flow Relating to Proved Oil and Gas Reserves [Abstract]
Beginning of year	$ 1,743,772	$ 1,523,681	$ 1,597,912
Sale of oil and natural gas produced, net of production costs	(502,928)	(418,224)	(346,627)
Net changes in prices and production costs	116,977	172,975	(206,640)
Extensions and discoveries	286,500	514,743	224,903
Improved recoveries	148,673	79,589	14,204
Changes in future development costs	(91,027)	(236,121)	(18,184)
Development costs incurred during the period that reduced future development costs	316,490	110,354	118,502
Revisions of previous quantity estimates	(40,481)	(95,855)	(192,894)
Purchases and sales of reserves in place, net	(278,825)	(20,021)	(54,070)
Accretion of discount	223,324	152,830	214,794
Net change in income taxes	(46,584)	(60,981)	166,238
Changes in production rates and other	18,809	20,802	5,543
End of year	$ 1,894,700	$ 1,743,772	$ 1,523,681