Derivative Activities - Combined Fair Value of Derivatives by Consolidated Balance Sheets (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Derivatives Fair Value [Line Items]
Derivative assets	$ 27,439	$ 153,267
Derivative liabilities	(28,032)	(7,934)
Natural gas | Swap
Derivatives Fair Value [Line Items]
Derivative assets	(12,701)	7,504
Derivative liabilities	(19,918)
Natural gas | Collars
Derivatives Fair Value [Line Items]
Derivative assets	33,552	122,255
Derivative liabilities	(6,206)	(3,463)
Natural gas | Basis Swaps
Derivatives Fair Value [Line Items]
Derivative assets		993
Crude oil | Swap
Derivatives Fair Value [Line Items]
Derivative assets	1,769	9,650
Derivative liabilities	2,929
Crude oil | Collars
Derivatives Fair Value [Line Items]
Derivative assets	(59)	2,222
Swap NGLs (Natural Gasoline) | C5 swaps
Derivatives Fair Value [Line Items]
Derivative assets	4,844	10,643
Derivative liabilities	3,467	2,275
Swap NGLs (Natural Gasoline) | C3 swaps
Derivatives Fair Value [Line Items]
Derivative assets	34
Derivative liabilities	$ (8,204)	$ (6,746)