Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2015	Nov. 01, 2015
Document and Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2015
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	Q3
Entity Registrant Name	FOSTER L B CO
Entity Central Index Key	0000352825
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding		10,320,151
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Well-known Seasoned Issuer	No
Trading Symbol	fstr