CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Operating revenues and other income:
Oil, gas, and NGL production revenue	$ 1,332,392	$ 836,288	$ 615,953
Realized hedge (loss) gain	(20,707)	23,465	140,648
Gain on divestiture activity (note 3)	220,676	155,277	11,444
Marketed gas system revenue	69,898	70,110	58,459
Other operating revenue	1,059	7,694	5,697
Total operating revenues and other income	1,603,318	1,092,834	832,201
Operating Expenses [Abstract]
Oil, gas, and NGL production expense	290,111	195,075	206,800
Depletion, depreciation, amortization, and asset retirement obligation liability accretion	511,103	336,141	304,201
Exploration	53,537	63,860	62,235
Impairment of proved properties	219,037	6,127	174,813
Abandonment and impairment of unproved properties	7,367	1,986	45,447
Inventory Write-down	0	0	14,223
General and administrative	118,526	106,663	76,036
Provision for Doubtful Accounts	0	0	(5,189)
Change in Net Profits Plan liability (note 8)	(25,477)	(34,441)	(7,075)
Unrealized and realized derivative (gain) loss (note 10)	(37,086)	8,899	20,469
Marketed gas system and other expense	64,249	66,726	57,587
Other expense	17,567	3,027	13,489
Total operating expenses	1,218,934	754,063	963,036
Income from operations	384,384	338,771	(130,835)
Nonoperating income (expense):
Interest income	466	321	227
Interest expense	(45,849)	(24,196)	(28,856)
Income before income taxes	339,001	314,896	(159,464)
Income tax expense	(123,585)	(118,059)	60,094
Net income	$ 215,416	$ 196,837	$ (99,370)
Basic weighted-average common shares outstanding (in shares)	63,755	62,969	62,457
Diluted weighted-average common shares outstanding (in shares)	67,564	64,689	62,457
Basic net income (loss) per common share (note 1)	$ 3.38	$ 3.13	$ (1.59)
Diluted net income (loss) per common share (note 1)	$ 3.19	$ 3.04	$ (1.59)