Supplemental Oil and Gas Disclosures (Principal Sources Chg SM) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Supplemental Oil And Gas Disclosures [Abstract]
Standarized measure, beginning of period	$ 535,682	$ 498,978	$ 264,872
Net changes in prices and production costs	(133,641)	(33,792)	(33,520)
Future development costs incurred	91,872	140,436	172,462
Net changes in estimated future development costs	134,810	(132,092)	(193,359)
Revisions of previous quantity estimates	(428,696)	88,008	(56,525)
Extensions and discoveries	43,783		102,386
Accretion of discount	65,895us-gaap_AccretionOfDiscount	78,151us-gaap_AccretionOfDiscount	42,589us-gaap_AccretionOfDiscount
Changes in income taxes, net	152,102	104,891	(74,778)
Sale of oil and gas produced, net of production costs	(226,799)	(232,220)	(165,547)
Purchased reserves			457,033
Sales of reserves in place		(4,074)	(6,971)
Change in production, timing and other	288,727	27,396	(9,664)
Standarized measure, end of period	$ 523,735	$ 535,682	$ 498,978

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Amount of increase (decrease) in standardized measure of discounted future net cash flow as a result of change in estimated future development costs.

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Amount of (increase) decrease in standardized measure of discounted future net cash flow as a result of previously estimated development costs incurred.

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Amount of increase (decrease) in standardized measure of discounted future net cash flow as a result of revisions of estimated quantity of oil and gas in proved reserves.

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Amount of (increase) decrease in standardized measure of discounted future net cash flow as a result of sales and transfers of oil and gas produced.

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Amount of standardized measure of discounted future net cash flow relating to proved oil and gas reserves.

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