Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Assets:
Current derivatives	$ 14,836	$ 18,987
Noncurrent derivatives	1,167
Liabilities:
Current derivatives	(7,262)	(3,549)
Noncurrent derivatives	(8,437)	(6,850)
Fair Value, Measurements, Recurring
Assets:
Assets of SERP	3,339	3,088
Liabilities:
Deferred compensation - SERP obligation and other	(3,409)	(3,168)
Assets and Liabilities, Fair Value Disclosure	234	8,508
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets:
Assets of SERP	3,339	3,088
Liabilities:
Deferred compensation - SERP obligation and other	(3,409)	(3,168)
Assets and Liabilities, Fair Value Disclosure	(70)	(80)
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Liabilities:
Assets and Liabilities, Fair Value Disclosure	304	8,588
Fair Value, Measurements, Recurring | Commodity contracts
Assets:
Current derivatives	14,492	18,987
Noncurrent derivatives	1,167
Liabilities:
Current derivatives	(7,262)	(3,549)
Noncurrent derivatives	(7,494)	(6,850)
Fair Value, Measurements, Recurring | Commodity contracts | Fair Value, Inputs, Level 2
Assets:
Current derivatives	14,492	18,987
Noncurrent derivatives	1,167
Liabilities:
Current derivatives	(7,262)	(3,549)
Noncurrent derivatives	(7,494)	(6,850)
Fair Value, Measurements, Recurring | Interest Rate Swap Contract
Assets:
Current derivatives	344
Liabilities:
Noncurrent derivatives	(943)
Fair Value, Measurements, Recurring | Interest Rate Swap Contract | Fair Value, Inputs, Level 2
Assets:
Current derivatives	344
Liabilities:
Noncurrent derivatives	$ (943)