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v3.19.1

Related Party Transactions - Narrative (Details) $ in Millions | 3 Months Ended |
--- | --- | ---
Mar. 31, 2019 USD ($) T | Mar. 31, 2018 USD ($) | Apr. 29, 2019 USD ($)
Related Party Transaction [Line Items] |  |  |
Ownership percentage by noncontrolling owners | 42.90% |  |
Economic ownership percentage by related party | 47.00% |  |
Related parties | $ 311.3 | $ 296.2 |
BHH [Member] |  |  |
Related Party Transaction [Line Items] |  |  |
Ownership percentage | 40.00% |  |
Glencore [Member] |  |  |
Related Party Transaction [Line Items] |  |  |
Related parties | $ 311.3 | $ 296.2 |
Consolidated Sales [Member] | Customer Concentration Risk [Member] | Glencore [Member] |  |  |
Related Party Transaction [Line Items] |  |  |
Major customer, percentage of revenue, net (percent) | 64.00% | 65.00% |
Subsequent Event [Member] | Glencore [Member] |  |  |
Related Party Transaction [Line Items] |  |  |
Due to related parties, noncurrent |  |  | $ 40.0
Aluminum [Member] | Glencore [Member] |  |  |
Related Party Transaction [Line Items] |  |  |
Related parties | $ 8.2 |  |
Number of metric tonnes | T | 21,865 |  |

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