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, 2013	Dec. 31, 2012	Dec. 31, 2011
Principal Sources of Change in Standardized Measure of Discounted Future Net Cash Flow Relating to Proved Oil and Gas Reserves [Abstract]
Beginning of year	$ 1,523,681	$ 1,597,912	$ 1,236,026
Sale of oil and natural gas produced, net of production costs	(418,224)	(346,627)	(374,300)
Net changes in prices and production costs	172,975	(206,640)	538,466
Extensions and discoveries	514,743	224,903	224,027
Improved recoveries	79,589	14,204	17,969
Changes in future development costs	(236,121)	(18,184)	(238,881)
Development costs incurred during the period that reduced future development costs	110,354	118,502	166,161
Revisions of previous quantity estimates	(95,855)	(192,894)	77,588
Purchases and sales of reserves in place, net	(20,021)	(54,070)	(40,662)
Accretion of discount	152,830	214,794	169,679
Net change in income taxes	(60,981)	166,238	(177,142)
Changes in production rates and other	20,802	5,543	(1,019)
End of year	$ 1,743,772	$ 1,523,681	$ 1,597,912