Fair Values of Derivative Instruments in Consolidated Balance Sheets (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Derivative [Line Items]
Derivatives designated as hedging instruments, Asset Derivatives	$ 544	$ 3,262
Total asset derivatives		2,437
Derivatives designated as hedging instruments, Liability Derivatives	8,670
Total liability derivatives		4,235
Commodity Forward Contracts
Derivative [Line Items]
Total asset derivatives	10,049
Total liability derivatives	2,884
Treasury Lock
Derivative [Line Items]
Total liability derivatives	813
Accounts Receivable
Derivative [Line Items]
Total asset derivatives	10,049	2,437
Accounts Receivable | Commodity Forward Contracts
Derivative [Line Items]
Derivatives designated as hedging instruments, Asset Derivatives	8,670
Derivatives not designated as hedging instruments, Asset Derivatives	1,379	2,437
Accrued Liabilities
Derivative [Line Items]
Total liability derivatives	3,697	4,235
Accrued Liabilities | Commodity Forward Contracts
Derivative [Line Items]
Derivatives designated as hedging instruments, Liability Derivatives	544	3,262
Derivatives not designated as hedging instruments, Liability Derivatives	2,340	973
Accrued Liabilities | Treasury Lock
Derivative [Line Items]
Derivatives designated as hedging instruments, Liability Derivatives	$ 813