| Document and Entity Information | Sep. 30, 2020 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Sep. 30, 2020 |
| Entity Registrant Name | KOPPERS HOLDINGS INC. |
| Entity Central Index Key | 0001315257 |
| Entity Emerging Growth Company | false |
| Title of 12(b) Security | Common Stock |
| Trading Symbol | KOP |
| Security Exchange Name | NYSE |
| Entity File Number | 1-32737 |
| 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 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
