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COVER
 9 Months Ended
Sep. 30, 2025
shares
Cover [Abstract]  
Document Type 10-Q
Document Quarterly Report true
Document Period End Date Sep. 30, 2025
Document Transition Report false
Entity File Number 001-35931
Registrant Name Constellium SE
Entity Incorporation, State or Country Code I0
Entity Tax Identification Number 98-0667516
Entity Address, Address Line One 300 East Lombard Street, Suite 1710
Entity Address, City or Town Baltimore,
Entity Address, State or Province MD
Entity Address, Postal Zip Code 21202
City Area Code (443)
Local Phone Number 420-7861
Title of 12(b) Security Ordinary Shares
Trading Symbol CSTM
Security Exchange Name NYSE
Entity Current Reporting Status Yes
Entity Interactive Data Current Yes
Entity Filer Category Large Accelerated Filer
Entity Small Business false
Entity Emerging Growth Company false
Entity Shell Company false
Entity Common Stock, Shares Outstanding 137,801,779
Current Fiscal Year End Date --12-31
Amendment Flag false
Document Fiscal Year Focus 2025
Document Fiscal Period Focus Q3
Central Index Key 0001563411