Mineral Leaseholds, Net - Summary of Mineral Leaseholds (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012	Dec. 31, 2012 Successor [Member]	Dec. 31, 2011 Successor [Member]
Schedule Of Leases [Line Items]
Mineral leaseholds	$ 1,423	$ 1,502	$ 1,502	$ 42
Less accumulated depletion	(102)	(63)	(63)	(4)
Net	$ 1,321	$ 1,439	$ 1,439	$ 38

Carrying amount at the balance sheet date of mineral properties, gross of adjustments.

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 19

-Paragraph 15

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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

-Name Accounting Principles Board Opinion (APB)

-Number 12

-Paragraph 5

-Subparagraph b

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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_MineralPropertiesGross
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant