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Related Party Transactions - Narrative (Details) ¥ in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($)	Dec. 31, 2019 CNY (¥)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	May 22, 2019	Apr. 29, 2019 USD ($)
Related Party Transaction [Line Items]
Glencore beneficial ownership	42.90%	42.90%
Glencore economic ownership	46.90%	46.90%
Major customer, percentage revenue, net	10.00%	10.00%
Net sales to Glencore	$ 1,191.6		$ 1,204.5	$ 1,198.1
BHH [Member]
Related Party Transaction [Line Items]
Loss on sale of equity investments	4.3
Hawesville [Member]
Related Party Transaction [Line Items]
Due to related parties, noncurrent						$ 40.0
Glencore [Member]
Related Party Transaction [Line Items]
Net sales to Glencore	1,191.6		$ 1,204.5	$ 1,198.1
Guangxi Qiangqiang Carbon Co [Member] | BHH [Member]
Related Party Transaction [Line Items]
Ownership percentage, equity method					40.00%
Consideration for the sale	20.8	¥ 144.9
Loss on sale of equity investments	$ 4.3
Customer Concentration Risk [Member] | Sales Revenue [Member] | Glencore [Member]
Related Party Transaction [Line Items]
Major customer, percentage revenue, net	65.00%	65.00%
Alumina [Member] | Glencore [Member]
Related Party Transaction [Line Items]
Net sales to Glencore	$ 28.7

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