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		9 Months Ended			12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013		Sep. 30, 2012	Dec. 31, 2012
Costs Incurred Oil And Gas Property Acquisition Exploration And Development Activities [Line Items]
Acreage purchases			$ 69,987	[1]		$ 188,843	[1]
Development			727,386	[1]		1,049,129	[1]
Exploration:
Drilling			173,298	[1]		309,816	[1]
Expense			47,331	[1]		65,758	[1]
Stock-based compensation expense	13,244	12,233	40,979		36,763
Gas gathering facilities:
Development			40,626	[1]		41,035	[1]
Subtotal			1,061,641	[1]		1,658,630	[1]
Asset retirement obligations			(964)	[1]		57,982	[1]
Total costs incurred			1,060,677	[1]		1,716,612	[1]
Exploration Expense
Exploration:
Stock-based compensation expense			$ 3,013	[1]		$ 4,049	[1]

[1] Includes cost incurred whether capitalized or expensed.