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Goodwill and Other Intangible Assets (Narrative) (Details) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Finite-Lived Intangible Assets, Gross $ 95,771us-gaap_FiniteLivedIntangibleAssetsGross $ 60,967us-gaap_FiniteLivedIntangibleAssetsGross  
Amortization of Intangible Assets 4,695us-gaap_AmortizationOfIntangibleAssets 3,112us-gaap_AmortizationOfIntangibleAssets 2,961us-gaap_AmortizationOfIntangibleAssets
Maximum [Member]      
Finite-Lived Intangible Asset, Useful Life 25 years    
Minimum [Member]      
Finite-Lived Intangible Asset, Useful Life 5 years    
Weighted Average [Member]      
Finite-Lived Intangible Asset, Useful Life 16 years    
Rail Products Segment [Member]      
Finite-Lived Intangible Assets, Gross 44,781us-gaap_FiniteLivedIntangibleAssetsGross
/ us-gaap_StatementBusinessSegmentsAxis
= fstr_RailProductsSegmentMember		 44,455us-gaap_FiniteLivedIntangibleAssetsGross
/ us-gaap_StatementBusinessSegmentsAxis
= fstr_RailProductsSegmentMember		  
Construction Products Segment [Member]      
Finite-Lived Intangible Assets, Gross 3,178us-gaap_FiniteLivedIntangibleAssetsGross
/ us-gaap_StatementBusinessSegmentsAxis
= fstr_ConstructionProductsSegmentMember		 1,830us-gaap_FiniteLivedIntangibleAssetsGross
/ us-gaap_StatementBusinessSegmentsAxis
= fstr_ConstructionProductsSegmentMember		  
Tubular Products Segments [Member]      
Finite-Lived Intangible Assets, Gross $ 47,812us-gaap_FiniteLivedIntangibleAssetsGross
/ us-gaap_StatementBusinessSegmentsAxis
= fstr_TubularProductsSegmentsMember		 $ 14,682us-gaap_FiniteLivedIntangibleAssetsGross
/ us-gaap_StatementBusinessSegmentsAxis
= fstr_TubularProductsSegmentsMember