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Business acquisitions - Acquisition of Mt. Holly smelter narrative other units (Details) t in Thousands	Dec. 01, 2014 employee t	Nov. 30, 2014
Mt. Holly Smelter [Member]
Business Acquisition [Line Items]
Remaining percentage acquired in step acquisition		50.30%
Percentage of ownership after step acquisition	100.00%
Employees | employee	600
Mt Holly primary aluminum capacity (tonnes) | t	231
Mt. Holly [Member] | Berkeley Aluminum Inc. [Member]
Business Acquisition [Line Items]
Ownership percentage		49.70%

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Percentage of ownership at the acquisition date as a result of the business combination.

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Percentage of voting equity interests acquired in a business combination achieved in stages, including equity interests in the acquiree held by the acquirer immediately before the acquisition date and acquired at the acquisition date.

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The parent entity's interest in net assets of the subsidiary, expressed as a percentage.

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