Condensed Consolidating Financial Information - Cash Flows (Details) - USD ($) $ in Millions	9 Months Ended
Sep. 30, 2020	Sep. 30, 2019
CASH FLOWS FROM OPERATING ACTIVITIES:
Net cash provided by (used in) operating activities	$ 33.3	$ (4.4)
Net cash provided by (used in) investing activities
Purchase of property, plant and equipment	(11.5)	(39.9)
Proceeds from sale of joint venture		10.5
Proceeds from sales of property, plant & equipment	0.1	0.0
Intercompany transactions	0.0	0.0
Net cash provided by (used in) investing activities	(11.4)	(29.4)
Net cash provided by (used in) financing activities
Repayment of debt	(250.0)	0.0
Proceeds from issuance of debt	243.8	0.0
Repayments on Hawesville term loan	(15.0)	0.0
Borrowing under Hawesville term loan	0.0	40.0
Borrowings under revolving credit facilities	173.6	314.6
Repayments under revolving credit facilities	(127.6)	(337.5)
Debt issuance costs	(1.1)	0.0
Debt retirement costs	(0.2)	0.0
Other short term borrowings		3.4
Repayment on other short-term borrowings	0.0	(3.4)
Issuance of common stock	0.0	0.3
Intercompany transactions	0.0	0.0
Net cash provided by (used in) financing activities	23.5	17.4
CHANGE IN CASH, CASH EQUIVALENTS AND RESTRICTED CASH	45.4	(16.4)
Cash, cash equivalents and restricted cash, beginning of period	39.7	39.7
Cash, cash equivalents and restricted cash, end of period	85.1	23.3
Consolidating Adjustments [Member]
CASH FLOWS FROM OPERATING ACTIVITIES:
Net cash provided by (used in) operating activities	0.0	0.0
Net cash provided by (used in) investing activities
Purchase of property, plant and equipment	0.0	0.0
Proceeds from sale of joint venture		0.0
Proceeds from sales of property, plant & equipment	0.0
Intercompany transactions	21.6	59.9
Net cash provided by (used in) investing activities	21.6	59.9
Net cash provided by (used in) financing activities
Repayment of debt	0.0
Proceeds from issuance of debt	0.0
Repayments on Hawesville term loan	0.0
Borrowing under Hawesville term loan		0.0
Borrowings under revolving credit facilities	0.0	0.0
Repayments under revolving credit facilities	0.0	0.0
Debt issuance costs	0.0
Debt retirement costs	0.0
Other short term borrowings		0.0
Repayment on other short-term borrowings		0.0
Issuance of common stock		0.0
Intercompany transactions	(21.6)	(59.9)
Net cash provided by (used in) financing activities	(21.6)	(59.9)
CHANGE IN CASH, CASH EQUIVALENTS AND RESTRICTED CASH	0.0	0.0
Cash, cash equivalents and restricted cash, beginning of period	0.0	0.0
Cash, cash equivalents and restricted cash, end of period	0.0	0.0
The Company [Member] \| Reportable Legal Entities [Member]
CASH FLOWS FROM OPERATING ACTIVITIES:
Net cash provided by (used in) operating activities	(26.1)	(57.1)
Net cash provided by (used in) investing activities
Purchase of property, plant and equipment	(0.8)	(0.6)
Proceeds from sale of joint venture		0.0
Proceeds from sales of property, plant & equipment	0.0
Intercompany transactions	14.9	(9.3)
Net cash provided by (used in) investing activities	14.1	(9.9)
Net cash provided by (used in) financing activities
Repayment of debt	(250.0)
Proceeds from issuance of debt	243.8
Repayments on Hawesville term loan	(15.0)
Borrowing under Hawesville term loan		40.0
Borrowings under revolving credit facilities	128.6	295.1
Repayments under revolving credit facilities	(127.6)	(318.0)
Debt issuance costs	(1.1)
Debt retirement costs	(0.2)
Other short term borrowings		3.4
Repayment on other short-term borrowings		(3.4)
Issuance of common stock		0.3
Intercompany transactions	36.3	51.0
Net cash provided by (used in) financing activities	14.8	68.4
CHANGE IN CASH, CASH EQUIVALENTS AND RESTRICTED CASH	2.8	1.4
Cash, cash equivalents and restricted cash, beginning of period	0.3	0.1
Cash, cash equivalents and restricted cash, end of period	3.1	1.5
Combined Guarantor Subsidiaries [Member] \| Reportable Legal Entities [Member]
CASH FLOWS FROM OPERATING ACTIVITIES:
Net cash provided by (used in) operating activities	21.3	45.0
Net cash provided by (used in) investing activities
Purchase of property, plant and equipment	(9.3)	(26.4)
Proceeds from sale of joint venture		0.0
Proceeds from sales of property, plant & equipment	0.0
Intercompany transactions	(38.3)	(52.6)
Net cash provided by (used in) investing activities	(47.6)	(79.0)
Net cash provided by (used in) financing activities
Repayment of debt	0.0
Proceeds from issuance of debt	0.0
Repayments on Hawesville term loan	0.0
Borrowing under Hawesville term loan		0.0
Borrowings under revolving credit facilities	0.0	0.0
Repayments under revolving credit facilities	0.0	0.0
Debt issuance costs	0.0
Debt retirement costs	0.0
Other short term borrowings		0.0
Repayment on other short-term borrowings		0.0
Issuance of common stock		0.0
Intercompany transactions	27.4	34.0
Net cash provided by (used in) financing activities	27.4	34.0
CHANGE IN CASH, CASH EQUIVALENTS AND RESTRICTED CASH	1.1	0.0
Cash, cash equivalents and restricted cash, beginning of period	0.9	0.8
Cash, cash equivalents and restricted cash, end of period	2.0	0.8
Combined Non-Guarantor Subsidiaries [Member] \| Reportable Legal Entities [Member]
CASH FLOWS FROM OPERATING ACTIVITIES:
Net cash provided by (used in) operating activities	38.1	7.7
Net cash provided by (used in) investing activities
Purchase of property, plant and equipment	(1.4)	(12.9)
Proceeds from sale of joint venture		10.5
Proceeds from sales of property, plant & equipment	0.1
Intercompany transactions	1.8	2.0
Net cash provided by (used in) investing activities	0.5	(0.4)
Net cash provided by (used in) financing activities
Repayment of debt	0.0
Proceeds from issuance of debt	0.0
Repayments on Hawesville term loan	0.0
Borrowing under Hawesville term loan		0.0
Borrowings under revolving credit facilities	45.0	19.5
Repayments under revolving credit facilities	0.0	(19.5)
Debt issuance costs	0.0
Debt retirement costs	0.0
Other short term borrowings		0.0
Repayment on other short-term borrowings		0.0
Issuance of common stock		0.0
Intercompany transactions	(42.1)	(25.1)
Net cash provided by (used in) financing activities	2.9	(25.1)
CHANGE IN CASH, CASH EQUIVALENTS AND RESTRICTED CASH	41.5	(17.8)
Cash, cash equivalents and restricted cash, beginning of period	38.5	38.8
Cash, cash equivalents and restricted cash, end of period	$ 80.0	$ 21.0

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