Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Assets:
Current derivatives	$ 11,252	$ 18,987
Noncurrent derivatives	6,892	0
Liabilities:
Current derivatives	(817)	(3,549)
Noncurrent derivatives	(1,677)	(6,850)
Fair Value, Measurements, Recurring
Assets:
Assets of SERP	3,200	3,088
Liabilities:
Deferred compensation - SERP obligation and other	(3,296)	(3,168)
Assets and Liabilities, Fair Value Disclosure	15,554	8,508
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets:
Assets of SERP	3,200	3,088
Liabilities:
Deferred compensation - SERP obligation and other	(3,296)	(3,168)
Assets and Liabilities, Fair Value Disclosure	(96)	(80)
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets:
Assets of SERP	0	0
Liabilities:
Deferred compensation - SERP obligation and other	0	0
Assets and Liabilities, Fair Value Disclosure	15,650	8,588
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 [Member]
Assets:
Assets of SERP	0	0
Liabilities:
Deferred compensation - SERP obligation and other	0	0
Assets and Liabilities, Fair Value Disclosure	0	0
Fair Value, Measurements, Recurring | Commodity contracts
Assets:
Current derivatives	11,252	18,987
Noncurrent derivatives	6,892
Liabilities:
Current derivatives	(817)	(3,549)
Noncurrent derivatives	(1,677)	(6,850)
Fair Value, Measurements, Recurring | Commodity contracts | Fair Value, Inputs, Level 1
Assets:
Current derivatives	0	0
Noncurrent derivatives	0
Liabilities:
Current derivatives	0	0
Noncurrent derivatives	0	0
Fair Value, Measurements, Recurring | Commodity contracts | Fair Value, Inputs, Level 2
Assets:
Current derivatives	11,252	18,987
Noncurrent derivatives	6,892
Liabilities:
Current derivatives	(817)	(3,549)
Noncurrent derivatives	(1,677)	(6,850)
Fair Value, Measurements, Recurring | Commodity contracts | Fair Value, Inputs, Level 3 [Member]
Assets:
Current derivatives	0	0
Noncurrent derivatives	0
Liabilities:
Current derivatives	0	0
Noncurrent derivatives	$ 0	$ 0