Fair Value Measurements - Value of Assets Measured at Fair Value on Non Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Impairment	$ 7,012	$ 1,281	$ 7,753	$ 1,281
Fair Value, Measurements, Nonrecurring | Natural Gas and Oil Properties
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair Value	500		500
Impairment	7,012		7,012
Fair Value, Measurements, Nonrecurring | Surface Property
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair Value		6,269		6,269
Impairment		$ 1,281	$ 741	$ 1,281