Cover - shares	6 Months Ended
	Jun. 30, 2021	Jul. 28, 2021
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Jun. 30, 2021
Document Transition Report	false
Commission File Number	000-10436
Entity Registrant Name	FOSTER L B CO
Entity Incorporation, State or Country Code	PA
Entity Tax Identification Number	25-1324733
Entity Address, Street Name	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 Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		10,834,481
Amendment Flag	false
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q2
Entity Central Index Key	0000352825
Current Fiscal Year End Date	--12-31