Costs Incurred for Property Acquisition, Exploration and Development (Detail) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended			12 Months Ended
		Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016		Jun. 30, 2015	Dec. 31, 2015
Costs Incurred Oil And Gas Property Acquisition Exploration And Development Activities [Line Items]
Acreage purchases	[1]			$ 8,873			$ 73,025
Development	[1]			219,386			708,268
Exploration:
Exploration drilling costs incurred	[1]			30,914			87,505
Stock-based compensation expense		$ 16,888	$ 18,411	29,795		$ 32,902
Gas gathering facilities:
Development	[1]			1,054			13,337
Subtotal	[1]			271,925			903,541
Asset retirement obligations				3,740	[1]	18,684	22,184	[1]
Total costs incurred	[1]			275,665			925,725
Expense
Exploration:
Exploration costs incurred	[1]			10,637			18,421
Stock-based compensation expense		$ 371	$ 751	$ 1,061	[1]	$ 1,483	$ 2,985	[1]

[1] Includes costs incurred whether capitalized or expensed.