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Business acquisitions - Acquisition of Mt. Holly aluminum smelter narrative (Details) t in Thousands, $ in Thousands |  | 1 Months Ended | 3 Months Ended | 12 Months Ended | 13 Months Ended
--- | --- | --- | --- | --- | ---
Dec. 01, 2014 USD ($) employee t | Apr. 30, 2015 USD ($) | Dec. 31, 2014 USD ($) | Jun. 30, 2015 USD ($) | Mar. 31, 2015 USD ($) | Dec. 31, 2014 USD ($) | Dec. 31, 2015 USD ($) | Dec. 31, 2014 USD ($) | Dec. 31, 2013 USD ($) | Dec. 31, 2015 USD ($)
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Gain on fair value of contingent consideration |  |  |  |  | $ 6,527 | $ 7,943 | $ 18,337 | $ 7,943 | $ 0 |
Gain on remeasurement of equity investment |  |  |  |  |  | 15,955 | $ 0 | 15,955 | $ 0 |
Mt. Holly Smelter [Member] |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Percentage of voting interests acquired | 50.30% |  |  |  |  |  |  |  |  |
Percentage of ownership after acquisition (percentage) |  |  |  |  |  |  | 100.00% |  |  | 100.00%
Employees | employee | 600 |  |  |  |  |  |  |  |  |
Primary aluminum capacity (in tonnes) | t | 231 |  |  |  |  |  |  |  |  |
Purchase price | $ 67,500 |  |  |  |  |  |  |  |  |
Acquisition-related costs |  |  | $ 1,087 |  |  | $ 1,087 | $ 452 | $ 1,087 |  | $ 452
Contingent consideration, maximum | 22,500 |  |  |  |  |  |  |  |  |
Contingent consideration arrangements | 12,500 |  |  |  |  |  |  |  |  |
Contingent consideration | 13,780 |  |  |  |  |  |  |  |  |
Gain on fair value of contingent consideration |  |  |  | $ 10,287 |  |  | 18,337 |  |  |
Contingent consideration receivable |  |  |  |  |  |  | $ 12,500 |  |  | 12,500
Consideration receivable, economic adjustment |  | $ 11,189 |  |  |  |  |  |  |  |
Consideration received for working capital adjustments |  | 124 |  |  |  |  |  |  |  |
Consideration received for expected future post-employment benefits | $ 2,400 |  |  |  |  |  |  |  |  |
Equity interest in acquiree (percentage) | 49.70% |  |  |  |  |  |  |  |  |
Fair value of equity interest in acquiree | $ 47,855 |  |  |  |  |  |  |  |  |
Gain on remeasurement of equity investment |  |  | $ 15,955 |  |  |  |  |  |  | $ 14,637
Pension funding total |  | $ 34,595 |  |  |  |  |  |  |  |
Due from Alumax, purchase price credit | $ 11,269 |  |  |  |  |  |  |  |  |

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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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