Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Assets:
Current derivatives	$ 26,763	$ 18,987
Noncurrent derivatives	12,664	0
Liabilities:
Current derivatives	(617)	(3,549)
Noncurrent derivatives	(1,651)	(6,850)
Fair Value, Measurements, Recurring
Assets:
Assets of SERP	3,298	3,088
Liabilities:
Deferred compensation - SERP obligation and other	(3,411)	(3,168)
Assets and Liabilities, Fair Value Disclosure	37,046	8,508
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets:
Assets of SERP	3,298	3,088
Liabilities:
Deferred compensation - SERP obligation and other	(3,411)	(3,168)
Assets and Liabilities, Fair Value Disclosure	(113)	(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	37,159	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	26,763	18,987
Noncurrent derivatives	12,664
Liabilities:
Current derivatives	(617)	(3,549)
Noncurrent derivatives	(1,651)	(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	26,763	18,987
Noncurrent derivatives	12,664
Liabilities:
Current derivatives	(617)	(3,549)
Noncurrent derivatives	(1,651)	(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