Commitments and Contingencies - Schedule of Future Payments of Legal Settlements (Details) - UPRR $ in Thousands	Dec. 31, 2022 USD ($)
Loss Contingencies [Line Items]
2023	$ 8,000
2024	8,000
Total	$ 16,000

Estimated Litigation Liability, Amount Awarded to Other Party, Payment Due Year One

No definition available.

Name:	fstr_EstimatedLitigationLiabilityAmountAwardedToOtherPartyPaymentDueYearOne
Namespace Prefix:	fstr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Estimated Litigation Liability, Amount Awarded to Other Party, Payment Due Year Two

+ References

No definition available.

+ Details

Name:	fstr_EstimatedLitigationLiabilityAmountAwardedToOtherPartyPaymentDueYearTwo
Namespace Prefix:	fstr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Aggregate carrying amount of the estimated litigation liability for known or estimated probable loss from litigation, which may include attorneys' fees and other litigation costs.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 1
-URI https://asc.fasb.org/extlink&oid=121557415&loc=d3e14326-108349

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.25)
-URI https://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

Name:	us-gaap_LitigationReserve
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- 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

- Details

Name:	srt_MajorCustomersAxis=fstr_UPRRMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: