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Related party transactions - Narrative (Details) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2017 | Mar. 31, 2016 | Dec. 31, 2015 USD ($) | Dec. 31, 2016 USD ($) t | Dec. 31, 2015 USD ($) | Dec. 31, 2014 USD ($)
Related Party Transaction [Line Items] |  |  |  |  |  |
Glencore beneficial ownership |  |  |  | 100.00% |  |
Major customer, percentage revenue, net |  |  |  | 10.00% |  |
BHH impairment | $ |  |  |  | $ 0 | $ 11,584 | $ 0
Glencore [Member] |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Glencore beneficial ownership | 47.50% |  |  | 47.50% |  |
BHH [Member] |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Ownership percentage, equity method | 40.00% |  |  | 40.00% |  |
BHH impairment | $ |  |  | $ 11,584 |  |  |
Glencore [Member] |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Other forward delivery contracts to sell primary aluminum (in tonnes) | t |  |  |  | 60,600 |  |
Customer Concentration Risk [Member] | Sales Revenue, Goods, Net [Member] | Glencore [Member] |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Major customer, percentage revenue, net | 77.00% | 88.00% |  | 89.00% |  |
Commodity Contract [Member] |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Open position to offset fixed prices | t |  |  |  | 9,531 |  |
Common stock [Member] | Glencore [Member] |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Glencore beneficial ownership | 42.90% |  |  | 42.90% |  |

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