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Related Party Transactions - Narrative (Details) $ in Millions | 3 Months Ended | 6 Months Ended | 7 Months Ended |  |
--- | --- | --- | --- | --- | ---
Jun. 30, 2019 USD ($) T | Jun. 30, 2018 USD ($) | Jun. 30, 2019 USD ($) T | Jun. 30, 2018 USD ($) | Dec. 31, 2019 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 | $ 305.0 | $ 282.7 | $ 616.4 | $ 579.0 |  |  |
Payments receivable | 10.6 |  | 10.6 |  |  |  |
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 | 305.0 | $ 282.7 | 616.4 | $ 579.0 |  |  |
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 | $ 13.6 |  | $ 21.8 |  |  |  |
Number of metric tonnes | T | 37,904 |  | 59,769 |  |  |  |
Consolidated Sales [Member] | Customer Concentration Risk [Member] | Glencore [Member] |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |
Major customer, percentage of revenue, net (percent) | 64.50% | 60.10% | 64.00% | 62.60% |  |  |
Forecast [Member] | BHH [Member] | GQQ [Member] |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |
Consideration |  |  |  |  | $ 144.9 |  |

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