CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Statement of Cash Flows [Abstract]
Net income (loss)	$ (54,249,000)	$ 215,416,000	$ 196,837,000
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
(Gain) loss on divestiture activity	27,018,000	(220,676,000)	(155,277,000)
Depletion, depreciation, amortization, and asset retirement obligation liability accretion	727,877,000	511,103,000	336,141,000
Exploratory dry hole expense	20,861,000	277,000	289,000
Impairment of proved properties	208,923,000	219,037,000	6,127,000
Abandonment and impairment of unproved properties	16,342,000	7,367,000	1,986,000
Stock-based compensation expense	30,185,000	26,824,000	26,743,000
Change in Net Profits Plan liability	(28,904,000)	(25,477,000)	(34,441,000)
Unrealized derivative (gain) loss	(11,366,000)	(62,757,000)	8,899,000
Amortization of debt discount and deferred financing costs	6,769,000	18,299,000	13,464,000
Deferred income taxes	(29,638,000)	123,789,000	114,517,000
Plugging and abandonment	(2,856,000)	(5,849,000)	(8,314,000)
Other	527,000	(6,027,000)	(3,993,000)
Changes in current assets and liabilities:
Accounts receivable	(21,389,000)	(41,998,000)	(47,153,000)
Refundable income taxes	2,217,000	2,901,000	24,291,000
Prepaid expenses and other	(1,484,000)	16,376,000	(35,363,000)
Accounts payable and accrued expenses	31,136,000	(18,073,000)	53,198,000
Excess income tax benefit from the exercise of stock awards	0	0	854,000
Net cash provided by operating activities	921,969,000	760,532,000	497,097,000
Cash flows from investing activities:
Net proceeds from sale of oil and gas properties	55,375,000	364,522,000	311,504,000
Capital expenditures	(1,507,828,000)	(1,633,093,000)	(668,288,000)
Acquisition of oil and gas properties	(5,773,000)	0	(664,000)
Other	893,000	3,661,000	(4,125,000)
Net cash used in investing activities	(1,457,333,000)	(1,264,910,000)	(361,573,000)
Cash flows from financing activities:
Proceeds from credit facility	1,609,000,000	322,000,000	571,559,000
Repayment of credit facility	(1,269,000,000)	(370,000,000)	(711,559,000)
Debt issuance costs related to credit facility	0	(8,719,000)	0
Net proceeds from 6.625% Senior Notes due 2019	0	341,122,000	0
Net Proceeds from 6.50% Senior Notes due 2021	0	343,120,000	0
Net Proceeds from 6.50% Senior Notes due 2023	392,138,000	0	0
Repayment of 3.50% Senior Convertible Notes	(287,500,000)	0	0
Proceeds from sale of common stock	5,816,000	7,327,000	6,440,000
Dividends paid	(6,511,000)	(6,382,000)	(6,297,000)
Net share settlement from issuance of stock awards	(21,622,000)	(9,973,000)	(2,093,000)
Excess income tax benefit from the exercise of stock awards	0	0	854,000
Other	(225,000)	0	0
Net cash provided by (used in) financing activities	422,096,000	618,495,000	(141,096,000)
Net change in cash and cash equivalents	(113,268,000)	114,117,000	(5,572,000)
Cash and cash equivalents at beginning of period	119,194,000	5,077,000	10,649,000
Cash and cash equivalents at end of period	5,926,000	119,194,000	5,077,000
Supplemental schedule of additional cash flow information and noncash investing and financing activities:
Cash paid for interest, net of capitalized interest	(51,328,000)	(22,133,000)	(13,340,000)
Net cash refunded for income taxes	1,389,000	4,046,000	25,578,000
Additions to oil and gas properties also included in accounts payable and accrued expenses	$ 262,800,000	$ 214,800,000	$ 238,500,000