CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Operating revenues and other income:
Oil, gas, and NGL production revenue	$ 362,595	$ 276,313
Realized hedge gain (loss) (note 10)	1,652	(1,375)
Gain on divestiture activity (note 3)	1,462	24,915
Marketed gas system and other operating revenue	11,714	15,476
Total operating revenues and other income	377,423	315,329
Operating expenses:
Oil, gas, and NGL production expense	87,132	65,812
Depletion, depreciation, amortization, and asset retirement obligation liability accretion	169,570	105,356
Exploration	18,607	12,712
Abandonment and impairment of unproved properties	142	3,079
General and administrative	28,142	25,861
Change in Net Profits Plan liability (note 7)	3,939	14,195
Unrealized and realized derivative (gain) loss (note 10)	2,216	88,429
Marketed gas system and other expense	11,450	19,857
Total operating expenses	321,198	335,301
Income from operations	56,225	(19,972)
Nonoperating income (expense):
Interest income	70	128
Interest expense	(14,278)	(9,714)
Income before income taxes	42,017	(29,558)
Income tax expense	(15,681)	11,055
Net income	$ 26,336	$ (18,503)
Basic weighted-average common shares outstanding (in shares)	64,104	63,447
Diluted weighted-average common shares outstanding (in shares)	67,845	63,447
Basic net income per common share (in dollars per share) (note 9)	$ 0.41	$ (0.29)
Diluted net income per common share (in dollars per share) (note 9)	$ 0.39	$ (0.29)
Cash dividends (in dollars per share)	$ 0.05	$ 0.05