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Related Party Transactions - Narrative (Details) ¥ in Millions, $ in Millions | 3 Months Ended | 7 Months Ended | 9 Months Ended |  |
--- | --- | --- | --- | --- | ---
Sep. 30, 2019 USD ($) t | Sep. 30, 2018 USD ($) | Dec. 31, 2019 CNY (¥) | Sep. 30, 2019 USD ($) t | Sep. 30, 2018 USD ($) | May 22, 2019 payment | Apr. 29, 2019 USD ($)
Related Party Transaction [Line Items] |  |  |  |  |  |  |
Ownership percentage by noncontrolling owners | 42.90% |  |  | 42.90% |  |  |
Economic ownership percentage by related party | 47.00% |  |  | 47.00% |  |  |
Related parties | $ 282.3 | $ 305.3 |  | $ 898.7 | $ 884.2 |  |
Payments receivable | 10.1 |  |  | 10.1 |  |  |
BHH [Member] | GQQ [Member] |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |
Ownership percentage |  |  |  |  |  | 40.00% |
Number of payments | payment |  |  |  |  |  | 2 |
Loss on sale of equity investments |  |  |  | 4.3 |  |  |
Glencore [Member] |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |
Related parties | 282.3 | $ 305.3 |  | 898.7 | $ 884.2 |  |
Hawesville [Member] |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |
Term loan - less current portion |  |  |  |  |  |  | $ 40.0
Aluminum [Member] | Glencore [Member] |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |
Related parties | $ 4.5 |  |  | $ 26.2 |  |  |
Number of metric tonnes | t | 12,737 |  |  | 72,506 |  |  |
Consolidated Sales [Member] | Customer Concentration Risk [Member] | Glencore [Member] |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |
Major customer, percentage of revenue, net (percent) | 64.50% | 63.40% |  | 64.10% | 62.90% |  |
Forecast [Member] | BHH [Member] | GQQ [Member] |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |
Consideration | ¥ |  |  | ¥ 144.9 |  |  |  |

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