Costs Incurred for Property Acquisition, Exploration and Development - Costs Incurred for Property Acquisition, Exploration and Development (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2013		Dec. 31, 2012
Costs Incurred Oil And Gas Property Acquisition Exploration And Development Activities [Line Items]
Acreage purchases	$ 9,394	[1]	$ 188,843	[1]
Development	309,240	[1]	1,049,129	[1]
Exploration:
Drilling	70,853	[1]	309,816	[1]
Expense	15,710	[1]	65,758	[1]
Stock-based compensation expense	1,070	[1]	4,049	[1]
Gas gathering facilities:
Development	6,789	[1]	41,035	[1]
Subtotal	413,056	[1]	1,658,630	[1]
Asset retirement obligations	1,690	[1]	57,982	[1]
Total costs incurred	$ 414,746	[1]	$ 1,716,612	[1]

[1] Includes cost incurred whether capitalized or expensed.