Fair Value Measurements - Valuation of Financial Instruments by Pricing Levels (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Derivative assets (liabilities):
Derivative assets (liabilities)	$ 88,981	$ 79,661
Fixed price swaps
Derivative assets (liabilities):
Derivative assets (liabilities)	69,091	62,599
Collars
Derivative assets (liabilities):
Derivative assets (liabilities)	20,720	21,816
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)	88,981	79,661
Fair Value, Inputs, Level 2 [Member] | Fixed price swaps
Derivative assets (liabilities):
Derivative assets (liabilities)	69,091	62,599
Fair Value, Inputs, Level 2 [Member] | Collars
Derivative assets (liabilities):
Derivative assets (liabilities)	20,720	21,816
Fair Value, Inputs, Level 2 [Member] | Call Option [Member]
Derivative assets (liabilities):
Derivative assets (liabilities)	(830)	(4,754)
Fair Value | Call Option [Member]
Derivative assets (liabilities):
Derivative assets (liabilities)	(830)	(4,754)
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