Segment and Geographic Area Information - Additional Information (Detail)	12 Months Ended
Dec. 31, 2014 Segment
Segment Reporting Information [Line Items]
Revenue percentage from aluminum and alumina	80.00%
Number of reportable segments	4us-gaap_NumberOfReportableSegments
Aluminum [Member]
Segment Reporting Information [Line Items]
Sales percentage	90.00%

Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

No definition available.

Name:	us-gaap_NumberOfReportableSegments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration