Derivative Activities - Fair Value of Derivative Contracts, by Consolidated Balance Sheets (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013		Dec. 31, 2012
Derivatives Fair Value [Line Items]
Assets Carrying Value	$ 75,075		$ 152,114
Liabilities Carrying Value	(16,308)		(12,963)
Net Carrying Value	58,767		139,151
Assets Carrying Value	71,425		15,086
Liabilities Carrying Value	(2,673)		(8,904)
Net Carrying Value	68,752		6,182
Swap
Derivatives Fair Value [Line Items]
Assets Carrying Value	11,028	[1]	22,236	[1]
Liabilities Carrying Value	(5,952)	[1]	(3,242)	[1]
Net Carrying Value	5,076	[1]	18,994	[1]
Collars
Derivatives Fair Value [Line Items]
Assets Carrying Value	64,047	[1]	129,878	[1]
Liabilities Carrying Value	(10,356)	[1]	(9,721)	[1]
Net Carrying Value	53,691	[1]	120,157	[1]
Assets Carrying Value	33,112	[1]	857	[1]
Liabilities Carrying Value	(440)	[1]
Net Carrying Value	32,672	[1]	857	[1]
Sold Swap
Derivatives Fair Value [Line Items]
Assets Carrying Value	36,505	[1]	7,316	[1]
Liabilities Carrying Value	(2,233)	[1]	(8,904)	[1]
Net Carrying Value	34,272	[1]	(1,588)	[1]
Re-purchased swaps
Derivatives Fair Value [Line Items]
Assets Carrying Value	1,808	[1]	5,920	[1]
Net Carrying Value	1,808	[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 derivatives in the accompanying consolidated balance sheets. See additional discussion above regarding the discontinuance of hedge accounting.