| Cover | Mar. 02, 2021 | Jul. 01, 2020 |
| Cover [Abstract] | ||
| Document Type | 8-K | |
| Document Period End Date | Mar. 2, 2021 | |
| Entity Registrant Name | L.B. Foster Company | |
| Entity Incorporation, State or Country Code | PA | |
| Entity File Number | 000-10436 | |
| Entity Tax Identification Number | 25-1324733 | |
| Entity Address, Postal Zip Code | 15220 | |
| Trading Symbol | FSTR | |
| Security Exchange Name | NASDAQ | |
| Title of 12(b) Security | Common Stock, Par Value $0.01 | |
| Entity Address, Address Line One | 415 Holiday Drive, Suite 100, | |
| Entity Address, City or Town | Pittsburgh, | |
| Entity Address, State or Province | PA | |
| City Area Code | 412 | |
| Local Phone Number | 928-3400 | |
| Written Communications | false | |
| Soliciting Material | false | |
| Pre-commencement Tender Offer | false | |
| Pre-commencement Issuer Tender Offer | false | |
| Entity Emerging Growth Company | false | |
| Entity Central Index Key | 0000352825 | |
| Amendment Flag | false |
