Discontinued Operations - Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Disposal Group, Including Discontinued Operation, Assets [Abstract]
Held-for-sale assets	$ 36.0	$ 46.8
Right-of-use assets	3.1	3.8
Disposal Group, Including Discontinued Operation, Liabilities [Abstract]
Accrued liabilities	1.5	1.6
Other current liabilities	5.6	5.1
Current liabilities	11.4	12.9
Discontinued operations | Superform Aluminum Superplastic
Disposal Group, Including Discontinued Operation, Assets [Abstract]
Accounts and other receivables	8.7	13.5
Inventory	12.6	16.9
Held-for-sale assets	21.3	30.4
Property, plant and equipment	7.9	8.7
Right-of-use assets	3.1	3.8
Other non-current assets
Total assets	32.3	42.9
Disposal Group, Including Discontinued Operation, Liabilities [Abstract]
Accounts payable	4.3	6.2
Accrued liabilities	1.5	1.6
Other current liabilities	1.5	0.6
Current liabilities	7.3	8.4
Other non-current liabilities	4.1	4.5
Total liabilities	$ 11.4	$ 12.9