Supplemental Condensed Consolidating Financial Information (Supplemental Condensed Consolidating Statement of Cash Flows) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Net cash provided by operating activities	$ 213,871	$ 171,252	$ 127,380
Property acquisitions	(578,372)	(235,486)	(623)
Exploration and development expenditures	(184,850)	(76,003)	(58,183)
Other property and equipment additions	(1,743)	(1,568)	(755)
Decrease in restricted cash		2,466	13,658
Net cash used in investing activities	(764,965)	(310,591)	(45,903)
Proceeds from indebtedness	509,313	203,794	20,394
Repayments of indebtedness	(20,000)		(94,882)
Deferred financing costs	(8,469)	(6,646)	(181)
Purchase of shares into treasury	(8,798)	(11,353)
Exercise of stock options	441	119
Net cash provided by (used) in financing activities	472,487	185,914	(74,669)
Net increase in cash and cash equivalents	(78,607)	46,575	6,808
Cash and cash equivalents at beginning of period	80,128	33,553	26,745
Cash and cash equivalents at end of period	1,521	80,128	33,553
Parent Company [Member]
Net cash provided by operating activities	190,851	136,245	84,613
Property acquisitions	(575,372)	(235,486)	(623)
Exploration and development expenditures	(165,308)	(40,996)	(15,416)
Other property and equipment additions	(1,265)	(1,568)	(755)
Decrease in restricted cash		2,466	13,658
Net cash used in investing activities	(741,945)	(275,584)	(3,136)
Proceeds from indebtedness	509,313	203,794	20,394
Repayments of indebtedness	(20,000)		(94,882)
Deferred financing costs	(8,469)	(6,646)	(181)
Purchase of shares into treasury	(8,798)	(11,353)
Exercise of stock options	441	119
Net cash provided by (used) in financing activities	472,487	185,914	(74,669)
Net increase in cash and cash equivalents	(78,607)	46,575	6,808
Cash and cash equivalents at beginning of period	80,128	33,553	26,745
Cash and cash equivalents at end of period	1,521	80,128	33,553
Guarantor Subsidiaries [Member]
Net cash provided by operating activities	23,020	35,007	42,767
Property acquisitions	(3,000)
Exploration and development expenditures	(19,542)	(35,007)	(42,767)
Other property and equipment additions	(478)
Net cash used in investing activities	$ (23,020)	$ (35,007)	$ (42,767)