Commitments and Contingent Liabilities - Future Payments (Details) $ in Thousands	3 Months Ended
Mar. 31, 2022 USD ($)
Commitments and Contingencies Disclosure [Abstract]
Remainder of 2022	$ 8,000
2023	8,000
2024	8,000
Total	$ 24,000

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

No definition available.

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