Derivative Instruments - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended	1 Months Ended
Jun. 30, 2012 6 5/8% Senior Notes Due 2016	Jul. 31, 2012 Refinancing of Debt Three Point Six Zero Percentage Senior Notes Due Twenty Twenty Two	Jul. 17, 2012 Refinancing of Debt Three Point Six Zero Percentage Senior Notes Due Twenty Twenty Two	Jun. 30, 2012 Commodity Forward Contracts Designated As Fair Value Hedges gal
Derivative [Line Items]
Feedstock forward contracts, in gallons				45,360,000
Senior notes, principal amount refinanced	$ 250,000
Stated interest rate	6.625%		3.60%
Maturity	2016	2022
Senior notes, principal amount issued			$ 250,000

The stated principal amount of the debt instrument at time of issuance, which may vary from the carrying amount because of unamortized premium or discount.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 55

-Paragraph 8

-URI http://asc.fasb.org/extlink&oid=6584090&loc=d3e28878-108400

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher AICPA

-Name Accounting Principles Board Opinion (APB)

-Number 21

-Paragraph 16, 20

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Name:	us-gaap_DebtInstrumentFaceAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant