Derivative Activities - Fair Value of Derivative Contracts, by Consolidated Balance Sheets (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012
Derivatives Fair Value [Line Items]
Assets Carrying Value	$ 107,022		$ 152,114
Liabilities Carrying Value	(16,996)		(12,963)
Net Carrying Value	90,026		139,151
Assets Carrying Value	11,623		15,086
Liabilities Carrying Value	(102,242)		(8,904)
Net Carrying Value	(90,619)		6,182
Swap
Derivatives Fair Value [Line Items]
Assets Carrying Value	15,431	[1]	22,236	[1]
Liabilities Carrying Value	(6,640)	[1]	(3,242)	[1]
Net Carrying Value	8,791	[1]	18,994	[1]
Collars
Derivatives Fair Value [Line Items]
Assets Carrying Value	91,591	[1]	129,878	[1]
Liabilities Carrying Value	(10,356)	[1]	(9,721)	[1]
Net Carrying Value	81,235	[1]	120,157	[1]
Assets Carrying Value	162	[1]	857	[1]
Liabilities Carrying Value	(54,110)	[1]
Net Carrying Value	(53,948)	[1]	857	[1]
Sold Swap
Derivatives Fair Value [Line Items]
Assets Carrying Value	6,702	[1]	7,316	[1]
Liabilities Carrying Value	(48,132)	[1]	(8,904)	[1]
Net Carrying Value	(41,430)	[1]	(1,588)	[1]
Re-purchased swaps
Derivatives Fair Value [Line Items]
Assets Carrying Value	4,759	[1]	5,920	[1]
Net Carrying Value	4,759	[1]	5,920	[1]
Basis Swap
Derivatives Fair Value [Line Items]
Assets Carrying Value			993	[1]
Net Carrying Value			$ 993	[1]

[1] Included in unrealized derivative gain or loss in the accompanying consolidated balance sheets.