Commitments and Contingent Liabilities (Future Payments) (Details) $ in Thousands	3 Months Ended
Mar. 31, 2019 USD ($)
Commitments and Contingencies Disclosure [Abstract]
Remainder of 2019	$ 8,000
2020	8,000
2021	8,000
2022	8,000
2023	8,000
2024	8,000
Total	$ 48,000

Litigation Settlement, Amount Awarded to Other Party, Scheduled Payment After Year Five

No definition available.

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

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

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

No definition available.

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

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

No definition available.

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

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

No definition available.

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

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
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

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