Supplemental Condensed Consolidating Financial Information (Supplemental Condensed Consolidating Statement of Cash Flows) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2013 Parent Company [Member]	Dec. 31, 2012 Parent Company [Member]	Dec. 31, 2011 Parent Company [Member]	Dec. 31, 2013 Guarantor Subsidiaries [Member]	Dec. 31, 2012 Guarantor Subsidiaries [Member]	Dec. 31, 2010 Guarantor Subsidiaries [Member]
Net cash provided by operating activities	$ 387,559	$ 213,871	$ 171,252	$ 356,597	$ 190,851	$ 136,245	$ 30,962	$ 23,020	$ 35,007
Decrease in restricted cash			2,466			2,466
Property acquisitions	(27,560)	(578,372)	(235,486)	(27,560)	(575,372)	(235,486)		(3,000)
Exploration and development expenditures	(322,040)	(184,850)	(76,003)	(284,073)	(165,308)	(40,996)	(37,967)	(19,542)	(35,007)
Deposit for Nexen Acquisition	(7,040)			(7,040)
Other property and equipment additions	(2,016)	(1,743)	(1,568)	(2,016)	(1,265)	(1,568)		(478)
Proceeds from sale of assets	52,317			45,312			7,005
Net cash used in investing activities	(306,339)	(764,965)	(310,591)	(275,377)	(741,945)	(275,584)	(30,962)	(23,020)	(35,007)
Proceeds from indebtedness		509,313	203,794		509,313	203,794
Repayments of indebtedness	(65,000)	(20,000)		(65,000)	(20,000)
Deferred financing costs	(674)	(8,469)	(6,646)	(674)	(8,469)	(6,646)
Purchase of shares into treasury	(9,640)	(8,798)	(11,353)	(9,640)	(8,798)	(11,353)
Exercise of stock options	1,385	441	119	1,385	441	119
Net cash provided by (used) in financing activities	(73,929)	472,487	185,914	(73,929)	472,487	185,914
Net increase in cash and cash equivalents	7,291	(78,607)	46,575	7,291	(78,607)	46,575
Cash and cash equivalents at beginning of period	1,521	80,128	33,553	1,521	80,128	33,553
Cash and cash equivalents at end of period	$ 8,812	$ 1,521	$ 80,128	$ 8,812	$ 1,521	$ 80,128