v3.25.2
COVER - shares
 6 Months Ended
Jun. 30, 2025
Dec. 31, 2024
Cover [Abstract]    
Document Type 10-Q  
Document Quarterly Report true  
Document Period End Date Jun. 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  
Common stock, shares issued (in shares) 146,819,884 146,819,884
Entity Common Stock, Shares Outstanding 139,539,023  
Current Fiscal Year End Date --12-31  
Amendment Flag false  
Document Fiscal Year Focus 2025  
Document Fiscal Period Focus Q2  
Central Index Key 0001563411