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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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