Condensed Consolidating Financial Statements (Condensed Balance Sheet) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Current Assets
Cash and cash equivalents	$ 79.1	$ 102.4	$ 74.6
Accounts receivable, net	364.2	245.7
Inventories	675.7	631.2
Prepaid expenses and other current assets	80.6	36.1
Intercompany receivables	0.0	0.0
Total Current Assets	1,199.6	1,015.4
Property, plant and equipment, net	1,470.5	1,470.9
Intangible assets, net	34.1	34.7
Deferred income taxes	69.5	70.7
Other long-term assets	55.1	52.7
Investments in subsidiaries	0.0	0.0
Total Assets	2,828.8	2,644.4
Current Liabilities
Accounts payable	341.6	299.2
Accrued liabilities	199.9	197.4
Current portion of long-term debt	10.3	9.1
Intercompany payables	0.0	0.0
Total Current Liabilities	551.8	505.7
Long-term debt	1,819.1	1,771.4
Deferred Revenue	74.0	17.0
Deferred income taxes	6.1	4.0
Accrued pension benefits	173.5	170.2
Accrued postretirement benefits	33.8	34.3
Other long-term liabilities	49.6	49.1
Total Long-Term Liabilities	2,156.1	2,046.0
Total equity	120.9	92.7
Total Liabilities and Equity	2,828.8	2,644.4
Eliminations
Current Assets
Cash and cash equivalents	(2.0)	(2.1)	(2.3)
Accounts receivable, net	0.0	0.0
Inventories	0.0	0.0
Prepaid expenses and other current assets	0.0	0.0
Intercompany receivables	(310.2)	(213.0)
Total Current Assets	(312.2)	(215.1)
Property, plant and equipment, net	0.0	0.0
Intangible assets, net	0.0	0.0
Deferred income taxes	0.0	0.0
Other long-term assets	0.0	0.0
Investments in subsidiaries	(1,649.2)	(1,615.2)
Total Assets	(1,961.4)	(1,830.3)
Current Liabilities
Accounts payable	(2.0)	(2.1)
Accrued liabilities	0.0	0.0
Current portion of long-term debt	0.0	0.0
Intercompany payables	(310.2)	(213.0)
Total Current Liabilities	(312.2)	(215.1)
Long-term debt	0.0	0.0
Deferred Revenue		0.0
Deferred income taxes		0.0
Accrued pension benefits		0.0
Accrued postretirement benefits		0.0
Other long-term liabilities		0.0
Total Long-Term Liabilities	0.0	0.0
Total equity	(1,649.2)	(1,615.2)
Total Liabilities and Equity	(1,961.4)	(1,830.3)
Aleris Corporation (Parent)
Current Assets
Cash and cash equivalents			0.0
Aleris Corporation (Parent) | Reportable Legal Entities
Current Assets
Cash and cash equivalents	0.0	0.0
Accounts receivable, net	0.0	0.0
Inventories	0.0	0.0
Prepaid expenses and other current assets	0.0	0.0
Intercompany receivables	0.0	0.0
Total Current Assets	0.0	0.0
Property, plant and equipment, net	0.0	0.0
Intangible assets, net	0.0	0.0
Deferred income taxes	0.0	0.0
Other long-term assets	0.0	0.0
Investments in subsidiaries	124.7	96.3
Total Assets	124.7	96.3
Current Liabilities
Accounts payable	0.0	0.0
Accrued liabilities	0.0	0.0
Current portion of long-term debt	0.0	0.0
Intercompany payables	3.8	3.6
Total Current Liabilities	3.8	3.6
Long-term debt	0.0	0.0
Deferred Revenue	0.0	0.0
Deferred income taxes	0.0	0.0
Accrued pension benefits	0.0	0.0
Accrued postretirement benefits	0.0	0.0
Other long-term liabilities	0.0	0.0
Total Long-Term Liabilities	0.0	0.0
Total equity	120.9	92.7
Total Liabilities and Equity	124.7	96.3
Aleris International, Inc.
Current Assets
Cash and cash equivalents			42.7
Aleris International, Inc. | Reportable Legal Entities
Current Assets
Cash and cash equivalents	34.7	40.3
Accounts receivable, net	0.1	0.2
Inventories	0.0	0.0
Prepaid expenses and other current assets	5.6	3.4
Intercompany receivables	223.5	134.6
Total Current Assets	263.9	178.5
Property, plant and equipment, net	0.0	0.0
Intangible assets, net	0.0	0.0
Deferred income taxes	0.0	0.0
Other long-term assets	3.2	3.4
Investments in subsidiaries	1,521.4	1,514.4
Total Assets	1,788.5	1,696.3
Current Liabilities
Accounts payable	4.0	4.3
Accrued liabilities	66.0	43.9
Current portion of long-term debt	0.0	0.0
Intercompany payables	71.3	54.8
Total Current Liabilities	141.3	103.0
Long-term debt	1,522.2	1,497.0
Deferred Revenue	0.0	0.0
Deferred income taxes	0.0	0.0
Accrued pension benefits	0.0	0.0
Accrued postretirement benefits	0.0	0.0
Other long-term liabilities	0.3	0.0
Total Long-Term Liabilities	1,522.5	1,497.0
Total equity	124.7	96.3
Total Liabilities and Equity	1,788.5	1,696.3
Guarantor Subsidiaries
Current Assets
Cash and cash equivalents			0.0
Guarantor Subsidiaries | Reportable Legal Entities
Current Assets
Cash and cash equivalents	0.0	0.0
Accounts receivable, net	147.2	83.0
Inventories	335.1	286.6
Prepaid expenses and other current assets	27.7	10.0
Intercompany receivables	85.8	46.0
Total Current Assets	595.8	425.6
Property, plant and equipment, net	891.2	903.7
Intangible assets, net	18.2	18.8
Deferred income taxes	0.0	0.0
Other long-term assets	8.4	8.0
Investments in subsidiaries	3.1	4.5
Total Assets	1,516.7	1,360.6
Current Liabilities
Accounts payable	169.8	140.7
Accrued liabilities	65.1	75.8
Current portion of long-term debt	0.9	1.0
Intercompany payables	204.5	137.8
Total Current Liabilities	440.3	355.3
Long-term debt	0.8	1.0
Deferred Revenue	74.0	17.0
Deferred income taxes	0.5	0.2
Accrued pension benefits	45.0	45.2
Accrued postretirement benefits	33.8	34.3
Other long-term liabilities	17.2	17.2
Total Long-Term Liabilities	171.3	114.9
Total equity	905.1	890.4
Total Liabilities and Equity	1,516.7	1,360.6
Non-Guarantor Subsidiaries
Current Assets
Cash and cash equivalents			$ 34.2
Non-Guarantor Subsidiaries | Reportable Legal Entities
Current Assets
Cash and cash equivalents	46.4	64.2
Accounts receivable, net	216.9	162.5
Inventories	340.6	344.6
Prepaid expenses and other current assets	47.3	22.7
Intercompany receivables	0.9	32.4
Total Current Assets	652.1	626.4
Property, plant and equipment, net	579.3	567.2
Intangible assets, net	15.9	15.9
Deferred income taxes	69.5	70.7
Other long-term assets	43.5	41.3
Investments in subsidiaries	0.0	0.0
Total Assets	1,360.3	1,321.5
Current Liabilities
Accounts payable	169.8	156.3
Accrued liabilities	68.8	77.7
Current portion of long-term debt	9.4	8.1
Intercompany payables	30.6	16.8
Total Current Liabilities	278.6	258.9
Long-term debt	296.1	273.4
Deferred Revenue	0.0	0.0
Deferred income taxes	5.6	3.8
Accrued pension benefits	128.5	125.0
Accrued postretirement benefits	0.0	0.0
Other long-term liabilities	32.1	31.9
Total Long-Term Liabilities	462.3	434.1
Total equity	619.4	628.5
Total Liabilities and Equity	$ 1,360.3	$ 1,321.5
7 7/8% Senior Notes | Senior Notes
Condensed Financial Statements, Captions [Line Items]
Stated interest rate	7.875%
9 1/2% Senior Notes | Senior Notes
Condensed Financial Statements, Captions [Line Items]
Stated interest rate	9.50%