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Commitments and Contingent Liabilities - Future Payments (Details) $ in Thousands | 9 Months Ended
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Sep. 30, 2021 USD ($)
Commitments and Contingencies Disclosure [Abstract] |
Remainder of 2021 | $ 4,000
2022 | 8,000
2023 | 8,000
2024 | 8,000
Total | $ 28,000

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

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- 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

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

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- References No definition available. + Details
Name: | us-gaap_CommitmentsAndContingenciesDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
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