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Business acquisitions - Acquisition of Mt. Holly smelter narrative other units (Details) - USD ($) $ in Thousands		1 Months Ended
	Dec. 01, 2014	Mar. 31, 2016	Apr. 30, 2015	Nov. 30, 2014
Business Acquisition [Line Items]
Payments to acquire businesses, net of cash acquired	$ 41,487
Mt. Holly Smelter [Member]
Business Acquisition [Line Items]
Percentage of ownership after step acquisition	100.00%
Purchase price	$ 67,500
Cash acquired in acquisition		$ 12,500
Consideration received for working capital adjustments			$ 11,313
Consideration received for expected future post-employment benefits			$ 2,400
Mt. Holly [Member] | Subsidiaries [Member]
Business Acquisition [Line Items]
Ownership percentage				49.70%

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The cash inflow associated with the acquisition of business during the period (for example, cash that was held by the acquired business).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Investing Activities
-URI http://asc.fasb.org/extlink&oid=6516133

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 12
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3179-108585

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 13
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-URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3213-108585

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-Name Accounting Standards Codification
-Glossary Investing Activities
-URI http://asc.fasb.org/extlink&oid=6516133

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