Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Assets:
Current derivatives	$ 18,987	$ 16,818
Noncurrent derivatives		3,889
Liabilities:
Current derivatives	(3,549)	(388)
Noncurrent derivatives	(6,850)
Fair Value, Measurements, Recurring
Assets:
Long-term investments - Rabbi Trust	3,088	6,440
Liabilities:
Deferred compensation - noncurrent	(3,168)	(6,948)
Totals	8,508	19,811
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets:
Long-term investments - Rabbi Trust	3,088	6,440
Liabilities:
Deferred compensation - noncurrent	(3,168)	(6,948)
Totals	(80)	(508)
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Liabilities:
Totals	8,588	20,319
Fair Value, Measurements, Recurring | Commodity contracts
Assets:
Current derivatives	18,987	15,075
Noncurrent derivatives		3,042
Liabilities:
Current derivatives	(3,549)	(388)
Noncurrent derivatives	(6,850)
Fair Value, Measurements, Recurring | Commodity contracts | Fair Value, Inputs, Level 2
Assets:
Current derivatives	18,987	15,075
Noncurrent derivatives		3,042
Liabilities:
Current derivatives	(3,549)	(388)
Noncurrent derivatives	(6,850)
Fair Value, Measurements, Recurring | Interest Rate Swap Contract
Assets:
Current derivatives		1,743
Noncurrent derivatives		847
Fair Value, Measurements, Recurring | Interest Rate Swap Contract | Fair Value, Inputs, Level 2
Assets:
Current derivatives		1,743
Noncurrent derivatives		$ 847