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Commitments and Contingencies - Schedule of Future Payments of Legal Settlements (Details) - Uprr - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2018
Loss Contingencies [Line Items] |  |
2021 | $ 8,000 |
2022 | 8,000 |
2023 | 8,000 |
2024 | 8,000 |
Total | $ 32,000 | $ 50,000

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 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

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- Definition Amount awarded to other party in judgment or settlement of litigation. + References No definition available. + Details
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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

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- Details
Name: | srt_MajorCustomersAxis=fstr_UprrMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |