Supplemental Information on Natural Gas and Oil Exploration, Development and Production Activities (Unaudited) - Costs Incurred for Property Acquisition, Exploration and Development (Parenthetical) (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]
Acquisition Costs, Period Cost	$ 404,252	[1],[2]
Asset retirement obligations	56,822	[1]	76,373	[1]	57,982	[1]
Conger Exchange Transaction
Costs Incurred Oil And Gas Property Acquisition Exploration And Development Activities [Line Items]
Asset retirement obligations	11,900
Gas Gathering Assets
Costs Incurred Oil And Gas Property Acquisition Exploration And Development Activities [Line Items]
Acquisition Costs, Period Cost	$ 134,800

[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.