CONSOLIDATED STATEMENTS OF OPERATIONS (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Operating revenues:
Oil, gas, and NGL production revenue	$ 2,481,544		$ 2,199,550		$ 1,473,868
Gain (loss) on divestiture activity	646		27,974		(27,018)
Marketed gas system revenue	24,897		60,039		52,808
Other operating revenues	15,220		5,811		5,444
Total operating revenues and other income	2,522,307		2,293,374		1,505,102
Operating Expenses
Oil, gas, and NGL production expense	715,878		597,045		391,872
Depletion, depreciation, amortization, and asset retirement obligation liability accretion	767,532		822,872		727,877
Exploration	129,857		74,104		90,248
Impairment of proved properties	84,480		172,641		208,923
Abandonment and impairment of unproved properties	75,638		46,105		16,342
General and administrative	167,103		149,551		119,815
Change in Net Profits Plan liability	(29,849)		(21,842)		(28,904)
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	583,264	[1]	3,080	[1]	55,630	[1]
Marketed gas system expense	24,460		57,647		47,583
Other operating expenses	4,658		30,076		6,993
Total operating expenses	1,356,493		1,925,119		1,525,119
Income (loss) from operations	1,165,814		368,255		(20,017)
Nonoperating income (expense):
Other, net	(2,561)		67		220
Interest expense	(98,554)		(89,711)		(63,720)
Income (loss) before income taxes	1,064,699		278,611		(83,517)
Income tax (expense) benefit	(398,648)		(107,676)		29,268
Net income (loss)	$ 666,051		$ 170,935		$ (54,249)
Basic weighted-average common shares outstanding	67,230		66,615		65,138
Diluted weighted-average common shares outstanding	68,044		67,998		65,138
Earnings Per Share, Basic [Abstract]
Basic net income (loss) per common share	$ 9.91		$ 2.57		$ (0.83)
Earnings Per Share, Diluted [Abstract]
Diluted net income (loss) per common share	$ 9.79		$ 2.51		$ (0.83)

[1] (2) Total derivative gain is reported in the derivative gain line item on the accompanying statements of cash flows within net cash provided by operating activities.