Long-Term Debt (Narrative) (Details) £ in Thousands, $ in Thousands		9 Months Ended
	Mar. 13, 2015 USD ($)	Sep. 30, 2015 GBP (£)	Sep. 30, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 23, 2014 USD ($)
Line of Credit Facility [Line Items]
Line of Credit Facility, Amount Outstanding			$ 203,460	$ 24,200
PNC Bank, N.A., Bank of America, N.A., Wells Fargo Bank, N.A. and Citizens Bank of Pennsylvania [Member] | Prior Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity					$ 200,000
PNC Bank, N.A., Bank of America, N.A., Wells Fargo Bank, N.A., Citizens Bank of Pennsylvania, and Branch Banking and Trust Company [Member] | Foreign Line of Credit [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 25,000
PNC Bank, N.A., Bank of America, N.A., Wells Fargo Bank, N.A., Citizens Bank of Pennsylvania, and Branch Banking and Trust Company [Member] | Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Initiation Date	Mar. 13, 2015
Line of Credit Facility, Maximum Borrowing Capacity	$ 335,000
Debt Instrument, Term	5 years
Line of Credit Facility, Borrowing Capacity, Description	The Amended Credit Agreement provides for a five-year, unsecured revolving credit facility that permits borrowings of up to $335,000 for the U.S. borrowers and a sublimit of the equivalent of $25,000 U.S. dollars that is available to the Canadian borrowers. The Amended Credit Agreement's accordion feature permits L.B. Foster to increase the available revolving borrowings under the facility by up to an additional $100,000 subject to L.B. Foster's receipt of increased commitments from existing or new lenders and to certain conditions being satisfied.
Line of Credit Facility, Provision for Increase in Capacity	$ 100,000
Line of Credit Facility, Interest Rate Description	Borrowings under the Amended Credit Agreement will bear interest at rates based upon either the base rate or Euro-rate plus applicable margins. Applicable margins are dictated by the ratio of L.B. Foster's indebtedness less consolidated cash on hand to L.B. Foster's consolidated EBITDA, as defined in the underlying Amended Credit Agreement. The base rate is the highest of (a) PNC Bank's prime rate, (b) the Federal Funds Rate plus 0.50% or (c) the daily Euro-rate (as defined in the Amended Credit Agreement) plus 1.00%. The base rate and Euro-rate spreads range from 0.00% to 1.50% and 1.00% to 2.50%, respectively.
Line of Credit Facility, Covenant Terms	The Amended Credit Agreement includes two financial covenants: (a) Leverage Ratio, defined as L.B. Foster's Indebtedness less consolidated cash on hand, in excess of $15,000, divided by L.B. Foster's consolidated EBITDA, which must not exceed 3.25 to 1.00 and (b) Minimum Interest Coverage, defined as consolidated EBITDA less Capital Expenditures divided by consolidated interest expense, which must be no less than 3.00 to 1.00.
Line of Credit Facility, Dividend Restrictions	The Amended Credit Agreement permits L.B. Foster to pay dividends, distributions, and make redemptions with respect to its stock provided no event of default or potential default (as defined in the Amended Credit Agreement) has occurred prior to or after giving effect to the dividend, distribution, or redemption. Dividends, distributions, and redemptions are capped at $25,000 per year when funds are drawn on the facility. If no drawings on the facility exist, dividends, distributions, and redemptions in excess of $25,000 per year are subjected to a limitation of $75,000 in the aggregate over the life of the facility. The $75,000 aggregate limitation also permits certain loans, investments, and acquisitions.
Line of Credit Facility, Amount Outstanding			526
Line of Credit Facility, Current Borrowing Capacity			131,014
Line of Credit Facility, Expiration Date	Mar. 13, 2020
NatWest Bank [Member] | Foreign Line of Credit [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		£ 1,500	2,269
Line of Credit Facility, Interest Rate Description		. The interest rate on this facility is the financial institution's base rate plus 1.50%.
Line of Credit Facility, Amount Outstanding			0
Line of Credit Facility, Current Borrowing Capacity			2,252
Line of Credit Facility, Collateral		This credit facility supports the subsidiary's working capital requirements and is collateralized by substantially all of the assets of its United Kingdom operations.
NatWest Bank Outstanding Guarantees [Member] | Foreign Line of Credit [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Amount Outstanding			$ 17