Segment Information (Reconciliation of Segment Income to Consolidated Statements of Comprehensive Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Segment Reporting Information [Line Items]
Depreciation and amortization	$ (34.7)	$ (25.7)
Restructuring charges	(0.9)	(0.4)
Interest expense, net	(33.8)	(27.1)
Unallocated gains (losses) on derivative financial instruments	33.9	(28.2)
Income (loss) before income taxes	10.0	(24.5)
Operating segments
Segment Reporting Information [Line Items]
Net income attributable to Aleris Corporation	72.1	63.6
Unallocated Amounts
Segment Reporting Information [Line Items]
Depreciation and amortization	(34.7)	(25.7)
Other corporate general and administrative expenses	(11.2)	(12.4)
Restructuring charges	(0.9)	(0.4)
Interest expense, net	(33.8)	(27.1)
Unallocated gains (losses) on derivative financial instruments	33.7	(7.7)
Unallocated currency exchange gains (losses)	1.2	(0.1)
Start-up costs	(16.0)	(14.5)
Other expense, net	$ (0.4)	$ (0.2)