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Business acquisitions - Summary of consideration transferred (Details) - USD ($) $ in Thousands		9 Months Ended
	Dec. 01, 2014	Sep. 30, 2015	Sep. 30, 2014
Business Acquisition [Line Items]
Purchase price		$ 0	$ 1,042
Mt. Holly Smelter [Member]
Business Acquisition [Line Items]
Purchase price	$ 67,500
Contingent consideration	13,780
Economic, working capital and other closing adjustments	(13,513)
Total consideration transferred	$ 67,767

X

- Definition

Amounts for the economic adjustment, working capital, partnership receivable, other post retirement employment benefits, taxes and business interruption coverage due to the entity from the former owners of the acquiree.

+ References

No definition available.

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

Amount of consideration transferred, consisting of acquisition-date fair value of assets transferred by the acquirer, liabilities incurred by the acquirer, and equity interest issued by the acquirer.

+ References

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



-Publisher FASB



-Name Accounting Standards Codification



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-SubTopic 30



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- Definition

Amount of liabilities incurred by the acquirer as part of consideration transferred in a business combination.

+ References

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-Publisher FASB



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- Definition

The cash outflow associated with the acquisition of business during the period. The cash portion only of the acquisition price.

+ References

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