Condensed Consolidating Financial Information - Balance Sheets (Details) - USD ($) $ in Millions	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Condensed Balance Sheet Statements, Captions [Line Items]
Cash & cash equivalents	$ 81.4		$ 38.9
Restricted cash	3.7		0.8
Accounts receivable - net	50.6		70.1
Due from affiliates	15.0		30.1
Inventories	267.5		320.6
Derivative assets	10.6		14.6
Prepaid and other current assets	14.5		12.2
Total current assets	443.3		487.3
Property, plant and equipment - net	899.7		949.2
Investment in subsidiaries	0.0		0.0
Due from affiliates - less current portion	1.3		0.5
Other assets	61.1		62.7
TOTAL	1,405.4		1,499.7
Accounts payable, trade	85.6		97.1
Due to affiliates	2.9		32.9
Accrued and other current liabilities	69.5		65.5
Accrued employee benefits costs	10.4		10.4
Hawesville term loan	20.0		20.0
U.S. revolving credit facility	5.0		4.0
Industrial revenue bonds	7.8		7.8
Total current liabilities	201.2		233.7
Senior notes payable	242.8		249.2
Hawesville term loan - less current portion	5.0		20.0
Iceland revolving credit facility	45.0		0.0
Accrued pension benefits costs - less current portion	56.0		60.8
Accrued postretirement benefits costs - less current portion	98.8		100.7
Due to affiliates - long term	0.0		0.0
Other liabilities	46.2		42.4
Leases - right of use liabilities	23.4		22.8
Deferred taxes	94.6		95.1
Total noncurrent liabilities	611.8		591.0
Preferred stock	0.0		0.0
Common stock	1.0		1.0
Other shareholders' equity	591.4		674.0
Total shareholders’ equity	592.4	$ 648.7	675.0	$ 689.9	$ 709.4	$ 762.2
TOTAL	1,405.4		1,499.7
Consolidating Adjustments [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Cash & cash equivalents	0.0		0.0
Restricted cash	0.0		0.0
Accounts receivable - net	0.0		0.0
Due from affiliates	0.0		0.0
Inventories	0.0		0.0
Derivative assets	0.0		0.0
Prepaid and other current assets	0.0		0.0
Total current assets	0.0		0.0
Property, plant and equipment - net	0.0		0.0
Investment in subsidiaries	(599.7)		(672.2)
Due from affiliates - less current portion	(1,355.3)		(1,292.0)
Other assets	0.0		0.0
TOTAL	(1,955.0)		(1,964.2)
Accounts payable, trade	0.0		0.0
Due to affiliates	0.0		0.0
Accrued and other current liabilities	0.0		0.0
Accrued employee benefits costs	0.0		0.0
Hawesville term loan	0.0		0.0
U.S. revolving credit facility	0.0
Industrial revenue bonds	0.0		0.0
Total current liabilities	0.0		0.0
Senior notes payable	0.0		0.0
Hawesville term loan - less current portion	0.0		0.0
Iceland revolving credit facility	0.0
Accrued pension benefits costs - less current portion	0.0		0.0
Accrued postretirement benefits costs - less current portion	0.0		0.0
Due to affiliates - long term	(1,355.3)		(1,292.0)
Other liabilities	0.0		0.0
Leases - right of use liabilities	0.0		0.0
Deferred taxes	0.0		0.0
Total noncurrent liabilities	(1,355.3)		(1,292.0)
Preferred stock	0.0		0.0
Common stock	0.0		(0.1)
Other shareholders' equity	(599.7)		(672.1)
Total shareholders’ equity	(599.7)		(672.2)
TOTAL	(1,955.0)		(1,964.2)
The Company [Member] | Reportable Legal Entities [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Cash & cash equivalents	3.1		0.3
Restricted cash	0.0		0.0
Accounts receivable - net	1.7		0.7
Due from affiliates	0.0		0.0
Inventories	0.0		0.0
Derivative assets	9.8		12.2
Prepaid and other current assets	4.1		3.0
Total current assets	18.7		16.2
Property, plant and equipment - net	14.8		17.2
Investment in subsidiaries	533.8		609.5
Due from affiliates - less current portion	768.7		749.5
Other assets	38.0		38.1
TOTAL	1,374.0		1,430.5
Accounts payable, trade	3.2		2.3
Due to affiliates	1.6		0.0
Accrued and other current liabilities	23.8		23.5
Accrued employee benefits costs	1.9		1.9
Hawesville term loan	20.0		20.0
U.S. revolving credit facility	5.0
Industrial revenue bonds	0.0		0.0
Total current liabilities	55.5		47.7
Senior notes payable	242.8		249.2
Hawesville term loan - less current portion	5.0		20.0
Iceland revolving credit facility	0.0
Accrued pension benefits costs - less current portion	23.9		25.1
Accrued postretirement benefits costs - less current portion	1.0		1.1
Due to affiliates - long term	439.8		403.4
Other liabilities	7.5		3.5
Leases - right of use liabilities	6.1		5.5
Deferred taxes	0.0		0.0
Total noncurrent liabilities	726.1		707.8
Preferred stock	0.0		0.0
Common stock	1.0		1.0
Other shareholders' equity	591.4		674.0
Total shareholders’ equity	592.4		675.0
TOTAL	1,374.0		1,430.5
Combined Guarantor Subsidiaries [Member] | Reportable Legal Entities [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Cash & cash equivalents	0.2		0.1
Restricted cash	1.8		0.8
Accounts receivable - net	46.8		64.1
Due from affiliates	4.5		10.9
Inventories	178.3		205.5
Derivative assets	0.0		0.0
Prepaid and other current assets	1.9		2.8
Total current assets	233.5		284.2
Property, plant and equipment - net	319.8		334.8
Investment in subsidiaries	65.9		62.7
Due from affiliates - less current portion	584.5		537.9
Other assets	4.2		5.7
TOTAL	1,207.9		1,225.3
Accounts payable, trade	59.4		66.2
Due to affiliates	1.3		2.2
Accrued and other current liabilities	22.3		27.3
Accrued employee benefits costs	7.7		7.7
Hawesville term loan	0.0		0.0
U.S. revolving credit facility	0.0
Industrial revenue bonds	7.8		7.8
Total current liabilities	98.5		111.2
Senior notes payable	0.0		0.0
Hawesville term loan - less current portion	0.0		0.0
Iceland revolving credit facility	0.0
Accrued pension benefits costs - less current portion	25.4		27.3
Accrued postretirement benefits costs - less current portion	96.0		97.8
Due to affiliates - long term	352.1		315.9
Other liabilities	24.4		22.2
Leases - right of use liabilities	0.0		0.4
Deferred taxes	0.4		0.4
Total noncurrent liabilities	498.3		464.0
Preferred stock	0.0		0.0
Common stock	0.0		0.0
Other shareholders' equity	611.1		650.1
Total shareholders’ equity	611.1		650.1
TOTAL	1,207.9		1,225.3
Combined Non-Guarantor Subsidiaries [Member] | Reportable Legal Entities [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Cash & cash equivalents	78.1		38.5
Restricted cash	1.9		0.0
Accounts receivable - net	2.1		5.3
Due from affiliates	10.5		19.2
Inventories	89.2		115.1
Derivative assets	0.8		2.4
Prepaid and other current assets	8.5		6.4
Total current assets	191.1		186.9
Property, plant and equipment - net	565.1		597.2
Investment in subsidiaries	0.0		0.0
Due from affiliates - less current portion	3.4		5.1
Other assets	18.9		18.9
TOTAL	778.5		808.1
Accounts payable, trade	23.0		28.6
Due to affiliates	0.0		30.7
Accrued and other current liabilities	23.4		14.7
Accrued employee benefits costs	0.8		0.8
Hawesville term loan	0.0		0.0
U.S. revolving credit facility	0.0
Industrial revenue bonds	0.0		0.0
Total current liabilities	47.2		74.8
Senior notes payable	0.0		0.0
Hawesville term loan - less current portion	0.0		0.0
Iceland revolving credit facility	45.0
Accrued pension benefits costs - less current portion	6.7		8.4
Accrued postretirement benefits costs - less current portion	1.8		1.8
Due to affiliates - long term	563.4		572.7
Other liabilities	14.3		16.7
Leases - right of use liabilities	17.3		16.9
Deferred taxes	94.2		94.7
Total noncurrent liabilities	742.7		711.2
Preferred stock	0.0		0.0
Common stock	0.0		0.1
Other shareholders' equity	(11.4)		22.0
Total shareholders’ equity	(11.4)		22.1
TOTAL	$ 778.5		$ 808.1