Fair Value Measurements - Valuation of Financial Instruments by Pricing Levels (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Derivative assets (liabilities):
Derivative assets (liabilities)	$ (3,816)	$ 101,193
Swap [Member]
Derivative assets (liabilities):
Derivative assets (liabilities)	13,418	104,426
Collars
Derivative assets (liabilities):
Derivative assets (liabilities)	(17,234)	(3,195)
Call Option [Member]
Derivative assets (liabilities):
Derivative assets (liabilities)	0	(38)
Fair Value, Inputs, Level 1 [Member]
Derivative assets (liabilities):
Derivative assets (liabilities)	0	0
Fair Value, Inputs, Level 1 [Member] | Swap [Member]
Derivative assets (liabilities):
Derivative assets (liabilities)	0	0
Fair Value, Inputs, Level 1 [Member] | Collars
Derivative assets (liabilities):
Derivative assets (liabilities)	0	0
Fair Value, Inputs, Level 1 [Member] | Call Option [Member]
Derivative assets (liabilities):
Derivative assets (liabilities)	0	0
Fair Value, Inputs, Level 2 [Member]
Derivative assets (liabilities):
Derivative assets (liabilities)	(3,816)	101,193
Fair Value, Inputs, Level 2 [Member] | Swap [Member]
Derivative assets (liabilities):
Derivative assets (liabilities)	13,418	104,426
Fair Value, Inputs, Level 2 [Member] | Collars
Derivative assets (liabilities):
Derivative assets (liabilities)	(17,234)	(3,195)
Fair Value, Inputs, Level 2 [Member] | Call Option [Member]
Derivative assets (liabilities):
Derivative assets (liabilities)	0	(38)
Fair Value, Inputs, Level 3 [Member]
Derivative assets (liabilities):
Derivative assets (liabilities)	0	0
Fair Value, Inputs, Level 3 [Member] | Swap [Member]
Derivative assets (liabilities):
Derivative assets (liabilities)	0	0
Fair Value, Inputs, Level 3 [Member] | Collars
Derivative assets (liabilities):
Derivative assets (liabilities)	0	0
Fair Value, Inputs, Level 3 [Member] | Call Option [Member]
Derivative assets (liabilities):
Derivative assets (liabilities)	$ 0	$ 0