Commitments and Contingent Liabilities - Future Payments (Details) $ in Thousands	9 Months Ended
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

Litigation Settlement Amount Awarded To Other Party Scheduled Payment Remainder Of Year

No definition available.

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Litigation Settlement, Amount Awarded to Other Party, Scheduled Payment Year One

No definition available.

Name:	fstr_LitigationSettlementAmountAwardedToOtherPartyScheduledPaymentYearOne
Namespace Prefix:	fstr_
Data Type:	xbrli:monetaryItemType
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Litigation Settlement, Amount Awarded to Other Party, Scheduled Payment Year Three

No definition available.

Name:	fstr_LitigationSettlementAmountAwardedToOtherPartyScheduledPaymentYearThree
Namespace Prefix:	fstr_
Data Type:	xbrli:monetaryItemType
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Litigation Settlement, Amount Awarded to Other Party, Scheduled Payment Year Two

No definition available.

Name:	fstr_LitigationSettlementAmountAwardedToOtherPartyScheduledPaymentYearTwo
Namespace Prefix:	fstr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

No definition available.

Name:	us-gaap_CommitmentsAndContingenciesDisclosureAbstract
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Amount awarded to other party in judgment or settlement of litigation.

No definition available.

Name:	us-gaap_LitigationSettlementAmountAwardedToOtherParty
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration