Fair Value Measurements - Valuation of Financial Instruments by Pricing Levels (Detail) (USD $)	Sep. 30, 2014	Dec. 31, 2013
Derivative assets (liabilities):
Derivative assets (liabilities)	$ 174,270,000	$ (94,748,000)
Fixed price swaps
Derivative assets (liabilities):
Derivative assets (liabilities)	151,896,000	(84,893,000)
Collars
Derivative assets (liabilities):
Derivative assets (liabilities)	22,374,000	(9,855,000)
Fair Value, Inputs, Level 1 [Member]
Derivative assets (liabilities):
Derivative assets (liabilities)	0	0
Fair Value, Inputs, Level 1 [Member] | Fixed price swaps
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 2 [Member]
Derivative assets (liabilities):
Derivative assets (liabilities)	174,270,000	(94,748,000)
Fair Value, Inputs, Level 2 [Member] | Fixed price swaps
Derivative assets (liabilities):
Derivative assets (liabilities)	151,896,000	(84,893,000)
Fair Value, Inputs, Level 2 [Member] | Collars
Derivative assets (liabilities):
Derivative assets (liabilities)	22,374,000	(9,855,000)
Fair Value, Inputs, Level 3 [Member]
Derivative assets (liabilities):
Derivative assets (liabilities)	0	0
Fair Value, Inputs, Level 3 [Member] | Fixed price swaps
Derivative assets (liabilities):
Derivative assets (liabilities)	0	0
Fair Value, Inputs, Level 3 [Member] | Collars
Derivative assets (liabilities):
Derivative assets (liabilities)	$ 0	$ 0