Condensed Consolidating Financial Statements (Condensed Income Statement) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Condensed Financial Statements, Captions [Line Items]
Revenues	$ 802.3	$ 674.2
Cost of sales	739.8	588.6
Gross profit	62.5	85.6
Selling, general and administrative expenses	50.6	52.9
Restructuring charges	(0.9)	(0.4)
(Gains) losses on derivative financial instruments	(33.9)	28.2
Other operating expense, net	0.7	1.0
Operating income	44.2	3.1
Interest expense, net	33.8	27.1
Other (income) expense, net	0.4	0.5
Equity in net earnings of affiliates	0.0	0.0
Income (loss) before income taxes	10.0	(24.5)
Provision for income taxes	5.4	10.7
Net income (loss)	4.6	(35.2)
Comprehensive (loss) income	25.1	(25.6)
Eliminations
Condensed Financial Statements, Captions [Line Items]
Revenues	(9.0)	(7.3)
Cost of sales	(9.0)	(7.3)
Gross profit	0.0	0.0
Selling, general and administrative expenses	0.0	0.0
Restructuring charges		0.0
(Gains) losses on derivative financial instruments	0.0	0.0
Other operating expense, net	0.0	0.0
Operating income	0.0	0.0
Interest expense, net	0.0	0.0
Other (income) expense, net	0.0	0.0
Equity in net earnings of affiliates	15.2	(70.3)
Income (loss) before income taxes	(15.2)	70.3
Provision for income taxes	0.0	0.0
Net income (loss)	(15.2)	70.3
Comprehensive (loss) income	(56.3)	51.0
Aleris Corporation (Parent) | Reportable Legal Entities
Condensed Financial Statements, Captions [Line Items]
Revenues	0.0	0.0
Cost of sales	0.0	0.0
Gross profit	0.0	0.0
Selling, general and administrative expenses	0.0	0.0
Restructuring charges	0.0	0.0
(Gains) losses on derivative financial instruments	0.0	0.0
Other operating expense, net	0.0	0.0
Operating income	0.0	0.0
Interest expense, net	0.0	0.0
Other (income) expense, net	0.0	0.0
Equity in net earnings of affiliates	(4.6)	35.2
Income (loss) before income taxes	4.6	(35.2)
Provision for income taxes	0.0	0.0
Net income (loss)	4.6	(35.2)
Comprehensive (loss) income	25.1	(25.6)
Aleris International, Inc. | Reportable Legal Entities
Condensed Financial Statements, Captions [Line Items]
Revenues	0.0	0.0
Cost of sales	0.0	0.0
Gross profit	0.0	0.0
Selling, general and administrative expenses	9.0	0.1
Restructuring charges	0.0	0.0
(Gains) losses on derivative financial instruments	0.0	0.0
Other operating expense, net	0.0	0.0
Operating income	(9.0)	(0.1)
Interest expense, net	0.0	0.0
Other (income) expense, net	(3.7)	(0.3)
Equity in net earnings of affiliates	(9.9)	35.3
Income (loss) before income taxes	4.6	(35.1)
Provision for income taxes	0.0	0.1
Net income (loss)	4.6	(35.2)
Comprehensive (loss) income	25.1	(25.6)
Guarantor Subsidiaries | Reportable Legal Entities
Condensed Financial Statements, Captions [Line Items]
Revenues	414.6	349.4
Cost of sales	390.4	324.9
Gross profit	24.2	24.5
Selling, general and administrative expenses	17.4	32.4
Restructuring charges	(0.7)	(0.3)
(Gains) losses on derivative financial instruments	(22.8)	20.0
Other operating expense, net	0.7	1.0
Operating income	28.2	(29.2)
Interest expense, net	26.3	20.7
Other (income) expense, net	2.7	(1.5)
Equity in net earnings of affiliates	(0.7)	(0.2)
Income (loss) before income taxes	(0.1)	(48.2)
Provision for income taxes	0.2	(0.1)
Net income (loss)	(0.3)	(48.1)
Comprehensive (loss) income	1.1	(47.6)
Non-Guarantor Subsidiaries | Reportable Legal Entities
Condensed Financial Statements, Captions [Line Items]
Revenues	396.7	332.1
Cost of sales	358.4	271.0
Gross profit	38.3	61.1
Selling, general and administrative expenses	24.2	20.4
Restructuring charges	(0.2)	(0.1)
(Gains) losses on derivative financial instruments	(11.1)	8.2
Other operating expense, net	0.0	0.0
Operating income	25.0	32.4
Interest expense, net	7.5	6.4
Other (income) expense, net	1.4	2.3
Equity in net earnings of affiliates	0.0	0.0
Income (loss) before income taxes	16.1	23.7
Provision for income taxes	5.2	10.7
Net income (loss)	10.9	13.0
Comprehensive (loss) income	$ 30.1	$ 22.2