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Leases - Narratives (Details) | 12 Months Ended
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Dec. 31, 2021
Lessee, Lease, Description |
Lessee lease renewal term | 12 years
Lessee, lease, termination period | 1 year
Minimum |
Lessee, Lease, Description |
Lessee remaining lease term | 2 years
Maximum |
Lessee, Lease, Description |
Lessee remaining lease term | 12 years

X
- Definition Lessee, Lease, Renewal Term + References No definition available. + Details
Name: | fstr_LesseeLeaseRenewalTerm
Namespace Prefix: | fstr_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Lessee, Lease, Termination Period + References No definition available. + Details
Name: | fstr_LesseeLeaseTerminationPeriod
Namespace Prefix: | fstr_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Lessee, Remaining Lease Term + References No definition available. + Details
Name: | fstr_LesseeRemainingLeaseTerm
Namespace Prefix: | fstr_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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

X
- Details
Name: | srt_RangeAxis=srt_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |