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Retirement Plans (Defined Contribution Plans) (Details) (USD $) In Thousands, unless otherwise specified | 3 Months Ended
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Mar. 31, 2015 | Mar. 31, 2014
Defined Benefit Plan Disclosure [Line Items] |  |
Defined Contribution Plan, Cost Recognized | $ 824 us-gaap_DefinedContributionPlanCostRecognized | $ 675 us-gaap_DefinedContributionPlanCostRecognized
United States Benefit Plan [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Defined Contribution Plan, Cost Recognized | 682 us-gaap_DefinedContributionPlanCostRecognized / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = fstr_UnitedStatesBenefitPlanMember | 573 us-gaap_DefinedContributionPlanCostRecognized / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = fstr_UnitedStatesBenefitPlanMember
Canada Benefit Plan [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Defined Contribution Plan, Cost Recognized | 59 us-gaap_DefinedContributionPlanCostRecognized / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = fstr_CanadaBenefitPlanMember | 64 us-gaap_DefinedContributionPlanCostRecognized / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = fstr_CanadaBenefitPlanMember
United Kingdom Benefit Plans [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Defined Contribution Plan, Cost Recognized | $ 83 us-gaap_DefinedContributionPlanCostRecognized / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = fstr_UnitedKingdomBenefitPlanMember | $ 38 us-gaap_DefinedContributionPlanCostRecognized / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = fstr_UnitedKingdomBenefitPlanMember

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- Definition The amount of the cost recognized during the period for defined contribution plans. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 70 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6414718&loc=d3e28014-114942 + Details
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