Condensed Consolidating Financial Statements (Condensed Cash Flow Statement) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Condensed Financial Statements, Captions [Line Items]
Net cash provided (used) by operating activities	$ (34.9)	$ (38.6)
Investing activities
Payments for property, plant and equipment	(30.1)	(62.6)
Disbursements of intercompany loans	0.0
Repayments from intercompany loans	0.0
Equity contributions in subsidiaries	0.0	0.0
Return of investment in subsidiaries	0.0	0.0
Other	(0.2)	(0.4)
Net cash used by investing activities	(30.3)	(63.0)
Financing activities
Proceeds from revolving credit facilities	133.9	159.5
Payments on revolving credit facilities	(90.0)	(294.5)
Proceeds from senior secured notes, inclusive of premiums	0.0	263.8
Payments on other long-term debt	(3.8)	(2.3)
Proceeds from intercompany loans	0.0
Repayments on intercompany loans	0.0
Debt issuance costs	0.0	(1.8)
Proceeds from intercompany equity contributions	0.0	0.0
Dividends paid	0.0	0.0
Other	0.0	(1.2)
Net cash provided by financing activities	40.1	123.5
Effect of exchange rate differences on cash, cash equivalents and restricted cash	1.9	0.6
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect	(23.2)	22.5
Cash, cash equivalents and restricted cash at beginning of period	108.0	55.6
Cash, cash equivalents and restricted cash at end of period	84.8	78.1
Cash and cash equivalents	79.1	74.6	$ 102.4
Restricted cash (included in “Prepaid expenses and other current assets”)	5.7	3.5
Aleris Corporation (Parent)
Financing activities
Cash and cash equivalents		0.0
Aleris International, Inc.
Financing activities
Cash and cash equivalents		42.7
Guarantor Subsidiaries
Financing activities
Cash and cash equivalents		0.0
Non-Guarantor Subsidiaries
Financing activities
Cash and cash equivalents		34.2
Reportable Legal Entities | Aleris Corporation (Parent)
Condensed Financial Statements, Captions [Line Items]
Net cash provided (used) by operating activities	0.1	1.1
Investing activities
Payments for property, plant and equipment	0.0	0.0
Disbursements of intercompany loans	0.0
Repayments from intercompany loans	0.0
Equity contributions in subsidiaries	0.0	0.0
Return of investment in subsidiaries	0.0	0.0
Other	0.0	0.0
Net cash used by investing activities	0.0	0.0
Financing activities
Proceeds from revolving credit facilities	0.0	0.0
Payments on revolving credit facilities	0.0	0.0
Proceeds from senior secured notes, inclusive of premiums		0.0
Payments on other long-term debt	0.0	0.0
Proceeds from intercompany loans	0.0
Repayments on intercompany loans	0.0
Debt issuance costs
Proceeds from intercompany equity contributions	0.0	0.0
Dividends paid	0.0	0.0
Other	(0.1)	(1.1)
Net cash provided by financing activities	(0.1)	(1.1)
Effect of exchange rate differences on cash, cash equivalents and restricted cash	0.0	0.0
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect	0.0	0.0
Cash, cash equivalents and restricted cash at beginning of period	0.0	0.0
Cash, cash equivalents and restricted cash at end of period	0.0	0.0
Cash and cash equivalents	0.0		0.0
Restricted cash (included in “Prepaid expenses and other current assets”)	0.0	0.0
Reportable Legal Entities | Aleris International, Inc.
Condensed Financial Statements, Captions [Line Items]
Net cash provided (used) by operating activities	(2.9)	303.5
Investing activities
Payments for property, plant and equipment	0.0	0.0
Disbursements of intercompany loans	0.0
Repayments from intercompany loans	0.0
Equity contributions in subsidiaries	(13.2)	(406.6)
Return of investment in subsidiaries	0.0	8.3
Other	0.0	0.0
Net cash used by investing activities	(13.2)	(398.3)
Financing activities
Proceeds from revolving credit facilities	80.0	55.0
Payments on revolving credit facilities	(55.0)	(185.0)
Proceeds from senior secured notes, inclusive of premiums		263.8
Payments on other long-term debt	0.0	0.0
Proceeds from intercompany loans	25.0
Repayments on intercompany loans	(39.5)
Debt issuance costs		(1.8)
Proceeds from intercompany equity contributions	0.0	0.0
Dividends paid	0.0	0.0
Other	0.0	0.0
Net cash provided by financing activities	10.5	132.0
Effect of exchange rate differences on cash, cash equivalents and restricted cash	0.0	0.0
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect	(5.6)	37.2
Cash, cash equivalents and restricted cash at beginning of period	40.3	5.5
Cash, cash equivalents and restricted cash at end of period	34.7	42.7
Cash and cash equivalents	34.7		40.3
Restricted cash (included in “Prepaid expenses and other current assets”)	0.0	0.0
Reportable Legal Entities | Guarantor Subsidiaries
Condensed Financial Statements, Captions [Line Items]
Net cash provided (used) by operating activities	6.9	(355.6)
Investing activities
Payments for property, plant and equipment	(17.8)	(52.7)
Disbursements of intercompany loans	0.0
Repayments from intercompany loans	0.0
Equity contributions in subsidiaries	0.0	0.0
Return of investment in subsidiaries	0.1	5.8
Other	(0.3)	(0.4)
Net cash used by investing activities	(18.0)	(47.3)
Financing activities
Proceeds from revolving credit facilities	0.0	0.0
Payments on revolving credit facilities	0.0	0.0
Proceeds from senior secured notes, inclusive of premiums		0.0
Payments on other long-term debt	(0.3)	(0.1)
Proceeds from intercompany loans	0.0
Repayments on intercompany loans	0.0
Debt issuance costs		0.0
Proceeds from intercompany equity contributions	13.2	402.6
Dividends paid	(2.0)	0.0
Other	0.2	0.4
Net cash provided by financing activities	11.1	402.9
Effect of exchange rate differences on cash, cash equivalents and restricted cash	0.0	0.0
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect	0.0	0.0
Cash, cash equivalents and restricted cash at beginning of period	0.0	0.0
Cash, cash equivalents and restricted cash at end of period	0.0	0.0
Cash and cash equivalents	0.0		0.0
Restricted cash (included in “Prepaid expenses and other current assets”)	0.0	0.0
Reportable Legal Entities | Non-Guarantor Subsidiaries
Condensed Financial Statements, Captions [Line Items]
Net cash provided (used) by operating activities	2.5	11.8
Investing activities
Payments for property, plant and equipment	(12.3)	(9.9)
Disbursements of intercompany loans	(25.0)
Repayments from intercompany loans	39.5
Equity contributions in subsidiaries	0.0	0.0
Return of investment in subsidiaries	0.0	0.0
Other	0.1	0.0
Net cash used by investing activities	2.3	(9.9)
Financing activities
Proceeds from revolving credit facilities	53.9	104.5
Payments on revolving credit facilities	(35.0)	(109.5)
Proceeds from senior secured notes, inclusive of premiums		0.0
Payments on other long-term debt	(3.5)	(2.2)
Proceeds from intercompany loans	0.0
Repayments on intercompany loans	0.0
Debt issuance costs		0.0
Proceeds from intercompany equity contributions	0.0	4.0
Dividends paid	(39.6)	(14.4)
Other	(0.1)	(0.5)
Net cash provided by financing activities	(24.3)	(18.1)
Effect of exchange rate differences on cash, cash equivalents and restricted cash	1.9	0.6
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect	(17.6)	(15.6)
Cash, cash equivalents and restricted cash at beginning of period	69.7	53.3
Cash, cash equivalents and restricted cash at end of period	52.1	37.7
Cash and cash equivalents	46.4		64.2
Restricted cash (included in “Prepaid expenses and other current assets”)	5.7	3.5
Eliminations
Condensed Financial Statements, Captions [Line Items]
Net cash provided (used) by operating activities	(41.5)	0.6
Investing activities
Payments for property, plant and equipment	0.0	0.0
Disbursements of intercompany loans	25.0
Repayments from intercompany loans	(39.5)
Equity contributions in subsidiaries	13.2	406.6
Return of investment in subsidiaries	(0.1)	(14.1)
Other	0.0	0.0
Net cash used by investing activities	(1.4)	392.5
Financing activities
Proceeds from revolving credit facilities	0.0	0.0
Payments on revolving credit facilities	0.0	0.0
Proceeds from senior secured notes, inclusive of premiums		0.0
Payments on other long-term debt	0.0	0.0
Proceeds from intercompany loans	(25.0)
Repayments on intercompany loans	39.5
Debt issuance costs		0.0
Proceeds from intercompany equity contributions	(13.2)	(406.6)
Dividends paid	41.6	14.4
Other	0.0	0.0
Net cash provided by financing activities	42.9	(392.2)
Effect of exchange rate differences on cash, cash equivalents and restricted cash	0.0	0.0
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect	0.0	0.9
Cash, cash equivalents and restricted cash at beginning of period	(2.0)	(3.2)
Cash, cash equivalents and restricted cash at end of period	(2.0)	(2.3)
Cash and cash equivalents	(2.0)	(2.3)	$ (2.1)
Restricted cash (included in “Prepaid expenses and other current assets”)	$ 0.0	$ 0.0