Fair Value Measurements - Valuation of Financial Instruments by Pricing Levels (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Derivative assets (liabilities):
Derivative assets (liabilities)	$ 161,235	$ 35,448
Fixed price swaps
Derivative assets (liabilities):
Derivative assets (liabilities)	160,297	36,716
Collars
Derivative assets (liabilities):
Derivative assets (liabilities)	938	(1,268)
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)	161,235	35,448
Fair Value, Inputs, Level 2 [Member] | Fixed price swaps
Derivative assets (liabilities):
Derivative assets (liabilities)	160,297	36,716
Fair Value, Inputs, Level 2 [Member] | Collars
Derivative assets (liabilities):
Derivative assets (liabilities)	938	(1,268)
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