Costs Incurred for Property Acquisition, Exploration and Development - Costs Incurred for Property Acquisition, Exploration and Development (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended		12 Months Ended
	Sep. 30, 2012		Dec. 31, 2011
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Acreage purchases	$ 173,718	[1]	$ 220,576	[1]
Development	855,650	[1]	1,007,049	[1]
Exploration:
Drilling	268,328	[1]	226,920	[1]
Expense	48,738	[1]	77,259	[1]
Stock-based compensation expense	3,047	[1]	4,108	[1]
Gas gathering facilities:
Development	32,520	[1]	53,387	[1]
Subtotal	1,382,001	[1]	1,589,299	[1]
Asset retirement obligations	29,695	[1]	24,061	[1]
Total - Continuing operations	1,411,696	[1]	1,613,360	[1]
Discontinued operations			3,241	[1]
Total costs incurred	$ 1,411,696	[1]	$ 1,616,601	[1]

[1] Includes cost incurred whether capitalized or expensed.