Condensed Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012
Cash flows from operating activities:
Net income	$ 98,616	$ 36,904
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation, depletion and amortization	99,858	51,826
Accretion of liability for asset retirement obligations	12,198	6,559
Unrealized loss (gain) on derivative contracts	(27,300)	(13,958)
Non-cash compensation	3,448	2,318
Deferred income taxes	56,291	21,654
Exploration expenditures	3,691	4,173
Impairments	2,171	5,708
Amortization of deferred financing costs and discount on debt	2,655	1,004
Gain on sale of assets	26,856
Other	5,420	3,401
Changes in operating assets and liabilities:
Trade accounts receivable	(10,260)	901
Prepaid expenses	(5,857)	1,820
Other assets	284	(78)
Accounts payable and accrued expenses	4,888	5,297
Asset retirement obligation settlements	(26,771)	(19,346)
Net cash provided by operating activities	192,476	108,183
Cash flows provided (used) in investing activities:
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)
Cash flows provided by (used in) financing activities:
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 (decrease) 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