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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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