Costs Incurred for Property Acquisition, Exploration and Development (Detail) - USD ($) $ in Thousands		3 Months Ended			12 Months Ended
		Mar. 31, 2018		Mar. 31, 2017	Dec. 31, 2017
Acquisitions:
Acreage purchases	[1]	$ 19,705			$ 62,075
Oil and gas properties	[1]				18,269
Development	[1]	225,033			1,177,526
Exploration:
Exploration drilling costs incurred	[1]	1,855			2,030
Stock-based compensation expense		25,538	[2]	$ 13,954
Gas gathering facilities:
Development	[1]	8,590			15,097
Subtotal	[1]	262,902			1,328,659
Asset retirement obligations		5,063	[1]	1,715	20,245	[1]
Total costs incurred	[1]	267,965			1,348,904
Expense
Exploration:
Exploration costs incurred	[1]	6,968			50,920
Stock-based compensation expense		$ 751	[1],[2]	$ 507	$ 2,742	[1]

[1] Includes costs incurred whether capitalized or expensed.

[2] Includes $18.2 million accelerated vesting of equity grants.