Supplemental Condensed Consolidating Financial Information (Supplemental Condensed Consolidating Statement of Cash Flows) (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012
Net cash provided by operating activities	$ 192,476	$ 108,183
Property acquisitions	(2,082)	(33,064)
Exploration and development expenditures	(152,104)	(85,133)
Other property and equipment additions	(1,263)	(1,145)
Proceeds from sale of assets	52,237
Increase in restricted cash	(51,757)
Net cash used in investing activities	(154,969)	(119,342)
Repayments of indebtedness	(30,000)
Deferred financing costs	(638)	(6)
Purchase of shares into treasury	(5,096)	(8,183)
Exercise of stock options	591	67
Net cash used in financing activities	(35,143)	(8,122)
Net increase in cash and cash equivalents	2,364	(19,281)
Cash and cash equivalents at beginning of period	1,521	80,128
Cash and cash equivalents at end of period	3,885	60,847
Parent Company [Member]
Net cash provided by operating activities	182,763	97,225
Property acquisitions	(2,082)	(33,064)
Exploration and development expenditures	(135,386)	(74,175)
Other property and equipment additions	(1,263)	(1,145)
Proceeds from sale of assets	45,232
Increase in restricted cash	(51,757)
Net cash used in investing activities	(145,256)	(108,384)
Repayments of indebtedness	(30,000)
Deferred financing costs	(638)	(6)
Purchase of shares into treasury	(5,096)	(8,183)
Exercise of stock options	591	67
Net cash used in financing activities	(35,143)	(8,122)
Net increase in cash and cash equivalents	2,364	(19,281)
Cash and cash equivalents at beginning of period	1,521	80,128
Cash and cash equivalents at end of period	3,885	60,847
Guarantor Subsidiaries [Member]
Net cash provided by operating activities	9,713	10,958
Exploration and development expenditures	(16,718)	(10,958)
Proceeds from sale of assets	7,005
Net cash used in investing activities	$ (9,713)	$ (10,958)