Commitments, Contingencies and Legal Matters - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	6 Months Ended
Jun. 30, 2013	Dec. 31, 2012
Commitments And Contingencies Disclosure [Abstract]
Total purchase commitments due through 2016	$ 388.9
Company's percentage of take-or-pay agreement with SMP	51.00%
Terms in which take-or-pay agreement may be terminated by either party	Upon two years written notice
Outstanding obligations related to standby letters of credit	$ 3.9	$ 4.1
Period of renewal for standby letter of credit	Annually

Amount of the unrecorded obligation to transfer funds in the future for fixed or minimum amounts or quantities of goods or services at fixed or minimum prices (for example, as in take-or-pay contracts or throughput contracts).

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 440

-SubTopic 10

-Section 50

-Paragraph 4

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6394976&loc=d3e25336-109308

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