Reconciliation of Assets and Liabilities Measured at Fair Value on Recurring Basis using Significant Unobservable Inputs (Level 3) (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010	Dec. 31, 2009
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance of Level 3, Beginning Balance	$ (222,554)		$ 37,854	$ 237,903
Total gains or losses (realized/unrealized)	40,907		(67,412)	141,744
Purchases			23,196
Settlements	60,127		(216,192)	(341,793)
Transfers	117,268	[1]	0	0
Balance of Level 3, Ending Balance	(4,252)		(222,554)	37,854
Commodity derivatives
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance of Level 3, Beginning Balance	(205,860)		46,153	246,648
Total gains or losses (realized/unrealized)	44,075		(50,872)	147,527
Purchases			23,196
Settlements	50,713		(224,337)	(348,022)
Transfers	106,820	[1]
Balance of Level 3, Ending Balance	(4,252)		(205,860)	46,153
Interest rate swaps
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance of Level 3, Beginning Balance	(16,694)		(8,299)	(8,745)
Total gains or losses (realized/unrealized)	(3,168)		(16,540)	(5,783)
Settlements	9,414		8,145	6,229
Transfers	10,448	[1]
Balance of Level 3, Ending Balance			$ (16,694)	$ (8,299)

[1] Fair values related to the Company's oil and natural gas fixed price swaps, natural gas collars and interest rate swap were transferred from Level 3 to Level 2 in the fourth quarter of 2011 due to enhancements to the Company's internal valuation process, including the use of observable inputs to assess the fair value. During the years ended December 31, 2010 and 2009, the Company did not have any transfers between Level 1, Level 2 or Level 3 fair value measurements. The Company's policy is to recognize transfers in and/or out of fair value hierarchy levels as of the end of the quarterly reporting period in which the event or change in circumstances causing the transfer occurred.