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Business acquisitions - Acquisition of Mt. Holly smelter narrative (Details) - USD ($) $ in Thousands |  | 1 Months Ended | 3 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Dec. 01, 2014 | Mar. 31, 2016 | Apr. 30, 2015 | Mar. 31, 2016 | Mar. 31, 2015 | Dec. 31, 2015 | Dec. 31, 2014
Business Acquisition [Line Items] |  |  |  |  |  |  |
Unrealized gain on fair value of contingent consideration |  |  |  | $ 0 | $ 6,527 |  |
Total net cash consideration paid | $ 41,487 |  |  |  |  |  |
Mt. Holly Smelter [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Purchase price | 67,500 |  |  |  |  |  |
Acquisition costs | 1,539 |  |  |  |  |  |
Contingent consideration at acquisition date | 13,780 |  |  |  |  |  |
Unrealized gain on fair value of contingent consideration |  |  |  | $ 6,527 |  | $ 18,337 | $ 7,943
Proceeds from acquisition - contingent consideration |  | $ 12,500 |  |  |  |  |
Consideration receivable, economic adjustment |  |  | $ 11,189 |  |  |  |
Consideration received for working capital adjustments |  |  | $ 124 |  |  |  |
Consideration due for expected future post-employment benefits | 200 |  |  |  |  |  |
Consideration received for expected future post-employment benefits | $ 2,400 |  |  |  |  |  |

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- Definition Amount of consideration received, consisting of the economic adjustment due to the entity from the former owners of the acquiree. The related adjustments include metal off-take and aluminum sales agreements, cash funding and management fee adjustments, as well as adjustments for inventory and transition services. + References No definition available. + Details
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