Derivative Financial Instruments (Changes in Fair Value of Derivative Contracts Recognized in Earnings) (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	9 Months Ended
Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Derivatives, Fair Value [Line Items]
Realized (gain) loss	$ 875	$ 917	$ 6,960	$ 9,239
Unrealized (gain) loss	23,784	(18,240)	7,461	(20,054)
Total	$ 24,659	$ (17,323)	$ 14,421	$ (10,815)

The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of open agreements to purchase or sell mineral resources, energy, and agricultural products at some future point held at each balance sheet date, that was included in earnings for the period.

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 95

-Paragraph 28

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3602-108585

Name:	us-gaap_UnrealizedGainLossOnCommodityContracts
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration