Description of the Plan (Details) - EBP 201	12 Months Ended
Dec. 31, 2025 USD ($) year
EBP, Description of Plan [Line Items]
Pretax, maximum annual compensation to total compensation, percentage	75.00%
Maximum after tax annual compensation to total compensation, percentage	100.00%
Matching percentage	100.00%
Participant compensation matched, percentage	6.00%
Discretionary profit-sharing contribution, service period	1 year
Maximum amount invested in company stock, percentage	0.15
Transfer of account balances, maximum percentage allowed to be invested in company stock	0.15
Vesting period	2 years
Retirement age \| year	65
Early retirement age \| year	55
Early retirement, years of service	5 years
Involuntary cash-out limit	$ 1,000
Withdrawal age \| year	59.5
Minimum hardship withdrawal limit	$ 500
Minimum borrowing, amount	1,000
Maximum borrowing, amount	$ 50,000
General loan, borrowing term	60 months
Primary residence, borrowing term	120 months
Basis spread on variable rate	1.00%
Minimum
EBP, Description of Plan [Line Items]
Participant loan, interest rate	4.25%
Maximum
EBP, Description of Plan [Line Items]
Participant loan, interest rate	9.50%

EBP, Discretionary Profit-Sharing Contribution, Service Period

No definition available.

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- Definition

EBP, Early Retirement Age

+ References

No definition available.

+ Details

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Balance Type:	na
Period Type:	duration

- Definition

EBP, Early Retirement, Years of Service

+ References

No definition available.

+ Details

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Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Minimum Vesting service period for company contribution and/ or investment earning to the participant in Company Matching Contributions, Fixed Rate Contributions, Discretionary Profit-Sharing Contributions, Qualified Non-Elective Contributions, and Disability Contributions.

+ References

No definition available.

+ Details

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Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

EBP, Involuntary Cash-out Limit

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

EBP, Minimum Hardship Withdrawal Limit

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

EBP, Note Receivable From Participant, Account, Primary Residence, Maximum Borrowing Term

+ References

No definition available.

+ Details

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Data Type:	xbrli:durationItemType
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Period Type:	duration

- Definition

EBP, Note Receivable From Participant, General Loan, Maximum Borrowing Term

+ References

No definition available.

+ Details

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Namespace Prefix:	fstr_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

EBP, Participant Contribution, Account Balance Transfer, Maximum Percentage Invested in Company Stock

+ References

No definition available.

+ Details

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Namespace Prefix:	fstr_
Data Type:	xbrli:pureItemType
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Period Type:	duration

- Definition

EBP, Participant Contribution, After Tax, Maximum Annual Contribution to Total Compensation, Percentage

+ References

No definition available.

+ Details

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Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

EBP, Participant Contribution, Maximum Amount Invested in Company Stock, Percentage

+ References

No definition available.

+ Details

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Namespace Prefix:	fstr_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	duration

- Definition

EBP, Participant Loan, Basis Spread On Variable Rate

+ References

No definition available.

+ Details

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Namespace Prefix:	fstr_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

EBP, Retirement Age

+ References

No definition available.

+ Details

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Namespace Prefix:	fstr_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

EBP, Withdrawal Age

+ References

No definition available.

+ Details

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Balance Type:	na
Period Type:	duration

- 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

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 962
-SubTopic 205
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478416/962-205-50-1

+ Details

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

- Definition

Percentage of employer matching contribution on eligible participant contribution to employee stock purchase, saving, or similar plan (employee benefit plan (EBP)).

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 962
-SubTopic 325
-Name Accounting Standards Codification
-Section 55
-Paragraph 17
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478873/962-325-55-17

+ Details

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Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of participant compensation eligible for employer matching contribution for participant in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)).

+ References

+ Details

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Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of maximum account balance that participant in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)) can borrow.

+ References

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of minimum account balance that participant in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)) can borrow.

+ References

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Percentage of maximum pretax annual compensation that participant in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)), is permitted to contribute to total pretax annual compensation.

+ References

+ Details

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Namespace Prefix:	us-gaap-ebp_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Interest rate on participant loan in schedule of asset held for investment of employee stock purchase, saving, or similar plan (employee benefit plan (EBP)).

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name Employee Retirement Income Security Act of 1974
-Number 2520
-Section 103
-Subsection 10
-Paragraph b
-Subparagraph (1)(i)(D)
-Publisher DOL-EBSA

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 962
-SubTopic 205
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6A-05(a)(Schedule I))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477804/962-205-S99-1

+ Details

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Balance Type:	na
Period Type:	duration

- Details

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Namespace Prefix:
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- Details

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