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v3.19.1

Commitments and Contingencies (Schedule of Future Payments of Legal Settlements) (Details) - Subsequent Event - Uprr $ in Thousands | Mar. 13, 2019 USD ($)
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Loss Contingencies [Line Items] |
2019 | $ 10,000
2020 | 8,000
2021 | 8,000
2022 | 8,000
2023 | 8,000
2024 | 8,000
Total | $ 50,000

X
- Definition Litigation Settlement, Amount Awarded to Other Party, Scheduled Payment After Year Five + References No definition available. + Details
Name: | fstr_LitigationSettlementAmountAwardedToOtherPartyScheduledPaymentAfterYearFive
Namespace Prefix: | fstr_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Litigation Settlement, Amount Awarded to Other Party, Scheduled Payment Remainder of Year + References No definition available. + Details
Name: | fstr_LitigationSettlementAmountAwardedToOtherPartyScheduledPaymentRemainderOfYear
Namespace Prefix: | fstr_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Litigation Settlement, Amount Awarded to Other Party, Scheduled Payment Year Five + References No definition available. + Details
Name: | fstr_LitigationSettlementAmountAwardedToOtherPartyScheduledPaymentYearFive
Namespace Prefix: | fstr_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Litigation Settlement, Amount Awarded to Other Party, Scheduled Payment Year Four + References No definition available. + Details
Name: | fstr_LitigationSettlementAmountAwardedToOtherPartyScheduledPaymentYearFour
Namespace Prefix: | fstr_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Litigation Settlement, Amount Awarded to Other Party, Scheduled Payment Year Three + References No definition available. + Details
Name: | fstr_LitigationSettlementAmountAwardedToOtherPartyScheduledPaymentYearThree
Namespace Prefix: | fstr_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Litigation Settlement, Amount Awarded to Other Party, Scheduled Payment Year Two + References No definition available. + Details
Name: | fstr_LitigationSettlementAmountAwardedToOtherPartyScheduledPaymentYearTwo
Namespace Prefix: | fstr_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount awarded to other party in judgment or settlement of litigation. + References No definition available. + Details
Name: | us-gaap_LitigationSettlementAmountAwardedToOtherParty
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_LossContingenciesLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_MajorCustomersAxis=fstr_UprrMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |