v3.26.1
COVER - shares
6 Months Ended
Jun. 30, 2026
Jul. 31, 2026
Cover [Abstract]    
Document Type 10-Q  
Document Quarterly Report true  
Document Period End Date Jun. 30, 2026  
Document Transition Report false  
Entity File Number 1-32737  
Entity Registrant Name KOPPERS HOLDINGS INC.  
Entity Incorporation, State or Country Code PA  
Entity Tax Identification Number 20-1878963  
Entity Address, Address Line One 436 Seventh Avenue  
Entity Address, City or Town Pittsburgh  
Entity Address, State or Province PA  
Entity Address, Postal Zip Code 15219  
City Area Code 412  
Local Phone Number 227-2001  
Title of 12(b) Security Common Stock  
Trading Symbol KOP  
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   18,911,568
Entity Central Index Key 0001315257  
Document Fiscal Year Focus 2026  
Document Fiscal Period Focus Q2  
Amendment Flag false  
Current Fiscal Year End Date --12-31