Fair Value Measurements - Value of Assets Measured at Fair Value on Non Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended				12 Months Ended
	Dec. 31, 2014	Jun. 30, 2014	Sep. 30, 2013	Jun. 30, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Impairment	$ 3,033	$ 24,991	$ 7,012	$ 741	$ 28,024	$ 7,753	$ 35,554
Fair Value, Measurements, Nonrecurring | Natural Gas and Oil Properties
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair Value	15,605				15,605	500	12,604
Impairment					28,024	7,012	34,273
Fair Value, Measurements, Nonrecurring | Surface Property
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair Value						5,550	6,269
Impairment						$ 741	$ 1,281