Cover - USD ($)	12 Months Ended
	Dec. 31, 2020	Feb. 24, 2021	Jun. 30, 2020
Cover [Abstract]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2020
Document Transition Report	false
Entity File Number	0-10436
Entity Registrant Name	FOSTER L B CO
Entity Incorporation, State	PA
Entity Tax Identification Number	25-1324733
Entity Address, Street Address	415 Holiday Drive
Entity Address, Suite	Suite 100
Entity Address, City	Pittsburgh
Entity Address, State	PA
Entity Address, Postal Zip Code	15220
City Area Code	412
Local Phone Number	928-3400
Title of Each Class	Common Stock, Par Value $0.01
Trading Symbol(s)	FSTR
Name of Each Exchange On Which Registered	NASDAQ
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Public Float			$ 126,070,586
Entity Common Stock, Shares Outstanding		10,738,899
Amendment Flag	false
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Entity Central Index Key	0000352825
Current Fiscal Year End Date	--12-31
ICFR Auditor Attestation Flag	true