Derivative Activities - Combined Fair Value of Derivatives, by Consolidated Balance Sheets (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Derivative assets:
Derivative assets	$ 152,489	$ 251,500
Derivative liabilities:
Derivative liabilities	(13,233)	(173)
Natural gas [Member] | Swaps [Member]
Derivative assets:
Derivative assets	(2,922)	54,162
Derivative liabilities:
Derivative liabilities	(2,764)
Natural gas [Member] | Collars [Member]
Derivative assets:
Derivative assets	116,531	228,228
Derivative liabilities:
Derivative liabilities	(5,603)
Crude oil [Member] | Swaps [Member]
Derivative assets:
Derivative assets	9,848	(263)
Crude oil [Member] | Collars [Member]
Derivative assets:
Derivative assets	(1,441)	(16,607)
Derivative liabilities:
Derivative liabilities	141
Crude oil [Member] | Call options [Member]
Derivative assets:
Derivative assets	(3,733)	(29,348)
Crude oil [Member] | Put options [Member]
Derivative assets:
Derivative assets	46
Derivative liabilities:
Derivative liabilities	31
NGL [Member] | C5 swaps [Member]
Derivative assets:
Derivative assets	33,864	15,328
Derivative liabilities:
Derivative liabilities	2,378	(173)
NGL [Member] | C3 swaps [Member]
Derivative assets:
Derivative assets	296
Derivative liabilities:
Derivative liabilities	$ (7,416)