CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (USD $)	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012
Statement of Cash Flows [Abstract]
Net income	$ 93,249,000	$ 51,225,000
Adjustments to reconcile net income to net cash provided by operating activities:
Gain (loss) on divestiture activity	5,706,000	(22,714,000)
Depletion, depreciation, amortization, and asset retirement obligation liability accretion	424,440,000	331,178,000
Exploratory dry hole expense	5,886,000	8,198,000
Impairment of proved properties	55,771,000	38,523,000
Abandonment and impairment of unproved properties	4,641,000	10,849,000
Stock-based compensation expense	18,068,000	12,372,000
Change in Net Profits Plan liability	(7,363,000)	(18,140,000)
Unrealized derivative gain	(40,615,000)	(74,014,000)
Amortization of debt discount and deferred financing costs	2,440,000	4,616,000
Deferred income taxes	56,239,000	30,215,000
Plugging and abandonment	(3,746,000)	(1,516,000)
Other	5,769,000	(867,000)
Changes in current assets and liabilities:
Accounts receivable	(59,284,000)	735,000
Refundable income taxes	648,000	2,978,000
Prepaid expenses and other	(680,000)	(4,759,000)
Accounts payable and accrued expenses	46,598,000	(4,019,000)
Net Cash Provided by Operating Activities	596,355,000	410,288,000
Cash flows from investing activities:
Net proceeds from sale of oil and gas properties	20,343,000	15,410,000
Capital expenditures	(733,992,000)	(705,366,000)
Acquisition of proved and unproved oil and gas properties	(59,201,000)	(5,312,000)
Other	(4,940,000)	111,000
Net cash used in investing activities	(777,790,000)	(695,157,000)
Cash flows from financing activities:
Proceeds from credit facility	516,500,000	802,500,000
Repayment of credit facility	(828,500,000)	(741,500,000)
Deferred financing costs related to credit facility	(3,444,000)	0
Net proceeds from 5.0% Senior Notes Due 2024	490,820,000	0
Net proceeds from 6.50% Senior Notes Due 2023	0	392,336,000
Repayment of 3.50% Convertible Notes	0	(287,500,000)
Proceeds from sale of common stock	3,652,000	2,888,000
Dividends paid	(3,314,000)	(3,208,000)
Other	(29,000)	343,000
Net cash used In financing activities	175,685,000	165,859,000
Net change in cash and cash equivalents	(5,750,000)	(119,010,000)
Cash and cash equivalents at beginning of period	176,000	184,000
Supplemental schedule of additional cash flow information and noncash investing and financing activities:
Cash paid for interest	(36,089,000)	(30,137,000)
Net cash refunded for income taxes	332,000	2,815,000
Capital expenditures incurred but not yet paid	$ 243,500,000	$ 226,000,000