Supplemental Information on Natural Gas and Oil Exploration, Development and Production Activities (Unaudited) - Costs Incurred for Property Acquisition, Exploration and Development (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Costs Incurred Oil And Gas Property Acquisition Exploration And Development Activities [Line Items]
Acquisitions	$ 404,252	[1],[2]
Acreage purchases	226,475	[1]	137,538	[1]	188,843	[1]
Development	1,119,896	[1]	938,668	[1]	1,049,129	[1]
Gas gathering facilities:
Development	13,137	[1]	47,086	[1]	41,035	[1]
Subtotal	2,008,233	[1]	1,377,443	[1]	1,658,630	[1]
Asset retirement obligations	56,822	[1]	76,373	[1]	57,982	[1]
Total costs incurred	2,065,055	[1]	1,453,816	[1]	1,716,612	[1]
Drilling
Exploration:
Exploration costs incurred	180,925	[1]	189,742	[1]	309,816	[1]
Expense
Exploration:
Exploration costs incurred	58,979	[1]	60,384	[1]	65,758	[1]
Stock-based compensation expense
Exploration:
Exploration costs incurred	$ 4,569	[1]	$ 4,025	[1]	$ 4,049	[1]

[1] Includes cost incurred whether capitalized or expensed.

[2] See also Note 3 for additional information related to the Conger Exchange which includes $134.8 million of gas gathering assets received in the exchange and $11.9 million of asset retirement obligations added in the exchange.