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Business acquisitions - Acquisition of Mt. Holly smelter narrative (Details) - USD ($) $ in Thousands |  | 1 Months Ended | 3 Months Ended | 6 Months Ended | 12 Months Ended
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Dec. 01, 2014 | Apr. 30, 2015 | Jun. 30, 2015 | Jun. 30, 2014 | Jun. 30, 2015 | Jun. 30, 2014 | Dec. 31, 2014
Business Acquisition [Line Items] |  |  |  |  |  |  |
Unrealized gain on fair value of contingent consideration |  |  | $ 10,287 | $ 0 | $ 16,814 | $ 0 |
Mt. Holly Smelter [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Purchase price | $ 67,500 |  |  |  |  |  |
Purchase price cash paid |  | $ 38,162 |  |  |  |  | $ 53,831
Acquisition costs |  |  | 417 |  | 417 |  | 1,087
Contingent consideration, high | 22,500 |  |  |  |  |  |
Contingent consideration, low | 12,500 |  |  |  |  |  |
Contingent consideration at acquisition date | 13,780 |  |  |  |  |  |
Unrealized gain on fair value of contingent consideration |  |  | $ 10,287 |  | $ 16,814 |  |
Consideration receivable, economic adjustment | 11,189 |  |  |  |  |  |
Consideration received for working capital adjustments | 124 |  |  |  |  |  |
Consideration received for expected future post-employment benefits | 2,400 |  |  |  |  |  |
Consideration receivable for tax reimbursement | 200 |  |  |  |  |  |
Pension funding total | 34,595 |  |  |  |  |  |
Pension funding for unfunded GAAP liability | 3,753 |  |  |  |  |  |
Pension funding for 414(I) liability | $ 30,842 |  |  |  |  |  |
Equity interest percentage, pre step acquisition | 49.70% |  |  |  |  |  |
Remeasurement of existing equity investment | $ 56,723 |  |  |  |  |  |
Gain on remeasurement of equity investment |  |  |  |  |  |  | $ 1,318

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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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