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Business acquisitions - Acquisition of Mt. Holly aluminum smelter narrative (Details) $ in Thousands |  | 1 Months Ended | 3 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | ---
Dec. 01, 2014 USD ($) employee t | Mar. 31, 2016 USD ($) | Apr. 30, 2015 USD ($) | Dec. 31, 2014 USD ($) | Jun. 30, 2015 USD ($) | Mar. 31, 2015 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Dec. 31, 2014 USD ($) | Nov. 30, 2014
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Gain on remeasurement of equity investment |  |  |  |  |  |  | $ 0 | $ 0 | $ 15,955 |
Gain on fair value of contingent consideration |  |  |  |  |  | $ 6,527 | $ 0 | $ 18,337 | $ 7,943 |
Payments to acquire businesses, net of cash acquired | $ 41,487 |  |  |  |  |  |  |  |  |
Mt. Holly Smelter [Member] |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Percentage of voting interests acquired |  |  |  |  |  |  |  |  |  | 50.30%
Percentage of ownership after acquisition (percentage) | 100.00% |  |  |  |  |  |  |  |  |
Employees | employee | 600 |  |  |  |  |  |  |  |  |
Primary aluminum capacity (in tonnes) | t | 231,000 |  |  |  |  |  |  |  |  |
Purchase price | $ 67,500 |  |  |  |  |  |  |  |  |
Gain on remeasurement of equity investment |  |  |  | $ 15,955 |  |  |  |  |  |
Acquisition-related costs | $ 1,539 |  |  |  |  |  |  |  |  |
Cash acquired in acquisition |  | $ 12,500 |  |  |  |  |  |  |  |
Consideration received for working capital adjustments |  |  | $ 11,313 |  |  |  |  |  |  |
Consideration received for expected future post-employment benefits |  |  | $ 2,400 |  |  |  |  |  |  |
Gain on fair value of contingent consideration |  |  |  |  | $ 10,287 |  |  |  |  |
Subsidiaries [Member] | Mt. Holly [Member] |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Ownership percentage |  |  |  |  |  |  |  |  |  | 49.70%

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