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Commitments and Contingent Liabilites (Narrative) (Details) (USD $) In Thousands, unless otherwise specified | 168 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2011 item | Mar. 31, 2015 | Dec. 31, 2014
Product Liability Contingency [Line Items] |  |  |
Concrete Ties Manufactured | 3,000,000 fstr_ConcreteTiesManufactured |  |
Standard Product Warranty Accrual |  | $ 10,344 us-gaap_StandardProductWarrantyAccrual | $ 11,500 us-gaap_StandardProductWarrantyAccrual
Environmental and litigation reserves |  | 6,773 us-gaap_AccrualForEnvironmentalLossContingencies | 3,344 us-gaap_AccrualForEnvironmentalLossContingencies
Reserve for Concrete Ties [Member] |  |  |
Product Liability Contingency [Line Items] |  |  |
Standard Product Warranty Accrual |  | $ 9,090 us-gaap_StandardProductWarrantyAccrual / us-gaap_ValuationAllowancesAndReservesTypeAxis = fstr_ReserveForConcreteTiesMember | $ 10,331 us-gaap_StandardProductWarrantyAccrual / us-gaap_ValuationAllowancesAndReservesTypeAxis = fstr_ReserveForConcreteTiesMember
The UPRR [Member] |  |  |
Product Liability Contingency [Line Items] |  |  |
Concrete Ties Manufactured | 1,600,000 fstr_ConcreteTiesManufactured / us-gaap_MajorCustomersAxis = fstr_UprrMember |  |

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- Definition Total costs accrued as of the balance sheet date for environmental loss contingencies. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 30 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13207-110859 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14326-108349 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 25 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6395460&loc=d3e13647-108346 + Details
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- Definition Amount as of the balance sheet date of the aggregate standard product warranty liability. Does not include the balance for the extended product warranty liability. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 460 -SubTopic 10 -Section 50 -Paragraph 8 -Subparagraph (c)(1),(c)(5) -URI http://asc.fasb.org/extlink&oid=6398077&loc=d3e12565-110249 + Details
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