CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Operating revenues and other income:
Oil, gas, and NGL production revenue	$ 1,473,868	$ 1,332,392	$ 836,288
Realized hedge gain (loss)	3,866	(20,707)	23,465
Gain (loss) on divestiture activity	(27,018)	220,676	155,277
Marketed gas system revenue	52,808	69,898	70,110
Other operating revenue	1,578	1,059	7,694
Total operating revenues and other income	1,505,102	1,603,318	1,092,834
Operating Expenses
Oil, gas, and NGL production expense	391,872	290,111	195,075
Depletion, depreciation, amortization, and asset retirement obligation liability accretion	727,877	511,103	336,141
Exploration	90,248	53,537	63,860
Impairment of proved properties	208,923	219,037	6,127
Abandonment and impairment of unproved properties	16,342	7,367	1,986
General and administrative	119,815	118,526	106,663
Change in Net Profits Plan liability	(28,904)	(25,477)	(34,441)
Unrealized and realized derivative (gain) loss	(55,630)	(37,086)	8,899
Marketed gas system expense	47,583	64,249	66,726
Other operating expense	6,993	17,567	3,027
Total operating expenses	1,525,119	1,218,934	754,063
Income (loss) from operations	(20,017)	384,384	338,771
Nonoperating income (expense):
Interest income	220	466	321
Interest expense	(63,720)	(45,849)	(24,196)
Income (loss) before income taxes	(83,517)	339,001	314,896
Income tax benefit (expense)	29,268	(123,585)	(118,059)
Net income (loss)	$ (54,249)	$ 215,416	$ 196,837
Basic weighted-average common shares outstanding	65,138	63,755	62,969
Diluted weighted-average common shares outstanding	65,138	67,564	64,689
Basic net income (loss) per common share	$ (0.83)	$ 3.38	$ 3.13
Diluted net income (loss) per common share	$ (0.83)	$ 3.19	$ 3.04