Supplemental Condensed Consolidating Financial Information (Supplemental Condensed Consolidating Statement of Cash Flows) (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012
Net cash provided by (used in) operating activities	$ 309,161	$ 162,620
Property acquisitions	(26,979)	(34,458)
Exploration and development expenditures	(252,125)	(135,950)
Deposit for Hilcorp Acqusition		(55,000)
Other property and equipment additions	(1,517)	(1,474)
Proceeds from sale of assets	52,237
Net cash provided by (used in) investing activities	(228,384)	(226,882)
Repayments of indebtedness	(70,000)
Deferred financing costs	(670)	(6)
Purchase of shares into treasury	(9,640)	(8,798)
Exercise of stock options	951	257
Net cash provided by (used in) financing activities	(79,359)	(8,547)
Net increase in cash and cash equivalents	1,418	(72,809)
Cash and cash equivalents at beginning of period	1,521	80,128
Cash and cash equivalents at end of period	2,939	7,319
Parent Company [Member]
Net cash provided by (used in) operating activities	282,260	151,193
Property acquisitions	(26,979)	(34,458)
Exploration and development expenditures	(218,219)	(124,523)
Deposit for Hilcorp Acqusition		(55,000)
Other property and equipment additions	(1,517)	(1,474)
Proceeds from sale of assets	45,232
Net cash provided by (used in) investing activities	(201,483)	(215,455)
Repayments of indebtedness	(70,000)
Deferred financing costs	(670)	(6)
Purchase of shares into treasury	(9,640)	(8,798)
Exercise of stock options	951	257
Net cash provided by (used in) financing activities	(79,359)	(8,547)
Net increase in cash and cash equivalents	1,418	(72,809)
Cash and cash equivalents at beginning of period	1,521	80,128
Cash and cash equivalents at end of period	2,939	7,319
Guarantor Subsidiaries [Member]
Net cash provided by (used in) operating activities	26,901	11,427
Exploration and development expenditures	(33,906)	(11,427)
Proceeds from sale of assets	7,005
Net cash provided by (used in) investing activities	$ (26,901)	$ (11,427)