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Debt - Narrative (Details)						1 Months Ended	3 Months Ended	9 Months Ended
	May 14, 2021	Apr. 14, 2021 USD ($)	Apr. 09, 2021 USD ($) $ / shares	Apr. 07, 2021 USD ($) $ / shares	Apr. 29, 2019 USD ($) payment	Apr. 30, 2021 USD ($)	Jun. 30, 2021 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Line of Credit Facility [Line Items]
Proceeds from issuance of debt								$ 250,000,000.0	$ 0
Loss on early extinguishment of debt								(24,700,000)	(1,000,000.0)
Proceeds from convertible notes								$ 86,300,000	$ 0
Senior notes [Member]
Line of Credit Facility [Line Items]
Loss on early extinguishment of debt						$ 24,700,000
Redemption premium						18,100,000
Write off of deferred financing cost						4,400,000
Debt discount						$ 2,200,000
Senior secured notes, 7.5% [Member] | Senior notes [Member]
Line of Credit Facility [Line Items]
Stated interest rate, percentage		7.50%						7.50%
Face amount		$ 250,000,000
Proceeds from issuance of debt		$ 245,200,000
Redemption price, percentage								100.00%
Redemption period								12 months
Percentage of outstanding amount, offer to purchase								101.00%
Senior secured notes, 7.5% [Member] | Senior notes [Member] | Level 2 [Member]
Line of Credit Facility [Line Items]
Fair value of debt instrument								$ 264,600,000
Senior secured notes, 12.0% [Member] | Senior notes [Member]
Line of Credit Facility [Line Items]
Stated interest rate, percentage						12.00%				12.00%
Redemption price, percentage	107.00%
Debt instrument, repurchased face amount						$ 195,900,000
Debt discount										$ 2,300,000
Convertible senior note [Member] | Call Option [Member]
Line of Credit Facility [Line Items]
Issuance cost				$ 5,700,000
Strike price (in dollars per share) | $ / shares				$ 18.74
Cap price (in dollars per share) | $ / shares				$ 22.95
Loss on derivative							$ 1,000,000
Convertible senior note [Member] | Senior notes [Member]
Line of Credit Facility [Line Items]
Face amount			$ 86,300,000
Percentage of principal amount redeemed			100.00%					100.00%
Proceeds from convertible notes			$ 83,700,000
Conversion ratio (in shares)			0.0533547
Conversion price (in dollars per share) | $ / shares			$ 18.74
Interest rate, cash percentage			2.75%
Threshold percentage of stock price trigger								130.00%
Threshold trading days								20 days
Consecutive trading days								30 days
Convertible senior note [Member] | Senior notes [Member] | Level 2 [Member]
Line of Credit Facility [Line Items]
Fair value of debt instrument								$ 87,500,000
Revolving credit facility [Member]
Line of Credit Facility [Line Items]
Stated interest rate, percentage								5.375%
Revolving credit facility [Member] | U.S. revolving credit facility [Member]
Line of Credit Facility [Line Items]
Stated interest rate, percentage								4.00%
Credit facility face amount								$ 175,000,000
Outstanding borrowings								17,300,000
Outstanding letters of credit issued								108,500,000
Credit facility maximum amount								175,000,000.0
Revolving credit facility								17,300,000		0
Revolving credit facility [Member] | U.S. letter of credit [Member]
Line of Credit Facility [Line Items]
Letter of credit sub-facility amount								110,000,000
Revolving credit facility [Member] | U.S. revolving credit facility, accordion [Member]
Line of Credit Facility [Line Items]
Credit facility face amount								$ 50,000,000
Revolving credit facility [Member] | Iceland revolving line of credit [Member]
Line of Credit Facility [Line Items]
Stated interest rate, percentage								3.088%
Outstanding borrowings								$ 30,000,000.0
Outstanding letters of credit issued								0
Credit facility maximum amount								50,000,000
Revolving credit facility								$ 30,000,000		$ 0
Hawesville [Member]
Line of Credit Facility [Line Items]
Due to related parties, noncurrent					$ 40,000,000
Debt instrument, payment terms, number of monthly payments | payment					24
London Interbank Offered Rate (LIBOR) [Member] | Hawesville [Member]
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate					5.375%

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