Stockholders' Equity (Schedule of Stockholders' Equity) (Details) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($)
Increase (Decrease) in Stockholders' Equity Disclosure [Roll Forward]
Total Equity at beginning of period	$ 92.7
Total Equity at end of period	120.9
Aleris Corporation
Increase (Decrease) in Stockholders' Equity Disclosure [Roll Forward]
Total Equity at beginning of period	92.7
Net income attributable to Aleris Corporation	4.6
Other comprehensive income	20.5
Stock-based compensation activity	0.3
Adoption of ASC 606	2.8
Total Equity at end of period	$ 120.9