Fair Value Measurements - Value of Assets Measured at Fair Value on Non Recurring Basis (Detail) (USD $)	3 Months Ended	9 Months Ended		3 Months Ended	9 Months Ended			12 Months Ended
	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2013 Fair Value, Measurements, Nonrecurring Natural Gas and Oil Properties	Sep. 30, 2014 Fair Value, Measurements, Nonrecurring Natural Gas and Oil Properties	Sep. 30, 2013 Fair Value, Measurements, Nonrecurring Natural Gas and Oil Properties	Sep. 30, 2013 Fair Value, Measurements, Nonrecurring Surface Property	Dec. 31, 2013 Fair Value, Measurements, Nonrecurring Surface Property
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair Value				$ 500,000	$ 18,086,000	$ 500,000	$ 5,550,000
Impairment	$ 7,012,000	$ 24,991,000	$ 7,753,000	$ 7,012,000	$ 24,991,000	$ 7,012,000	$ 741,000	$ 741,000