Fair Value Measurements - Value of Assets Measured at Fair Value on Non Recurring Basis (Detail) - USD ($)	3 Months Ended				12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Dec. 31, 2014	Jun. 30, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Impairment of proved properties and other assets	$ 87,941,000	$ 502,233,000	$ 3,033,000	$ 24,991,000	$ 590,174,000	$ 28,024,000	$ 7,753,000
Fair Value, Measurements, Nonrecurring | Natural Gas and Oil Properties
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair Value					152,230,000	15,605,000	500,000
Impairment of proved properties and other assets					590,174,000	28,024,000	7,012,000
Fair Value, Measurements, Nonrecurring | Surface Property
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair Value					0	0	5,550,000
Impairment of proved properties and other assets					$ 0	$ 0	$ 741,000