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Label | Element | Value
--- | --- | ---
Accounting Standards Update 2018-02 [Member] | AOCI Attributable to Parent [Member] |  |
Tax Cuts And Jobs Act Of 2017 Reclassification From Aoci To Retained Earnings Tax Effect | fstr_TaxCutsAndJobsActOf2017ReclassificationFromAociToRetainedEarningsTaxEffect | $ (633,000)
Accounting Standards Update 2018-02 [Member] | Retained Earnings [Member] |  |
Tax Cuts And Jobs Act Of 2017 Reclassification From Aoci To Retained Earnings Tax Effect | fstr_TaxCutsAndJobsActOf2017ReclassificationFromAociToRetainedEarningsTaxEffect | 633,000
Accounting Standards Update 2016-06 [Member] |  |
Cumulative Effect of New Accounting Principle in Period of Adoption | us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption | (305,000)
Accounting Standards Update 2016-06 [Member] | Retained Earnings [Member] |  |
Cumulative Effect of New Accounting Principle in Period of Adoption | us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption | $ (305,000)

X
- Definition Tax Cuts And Jobs Act Of 2017 Reclassification From Aoci To Retained Earnings Tax Effect + References No definition available. + Details
Name: | fstr_TaxCutsAndJobsActOf2017ReclassificationFromAociToRetainedEarningsTaxEffect
Namespace Prefix: | fstr_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount after tax of increase (decrease) to equity or (increase) decrease to net assets, resulting from the cumulative effect adjustment of a new accounting principle applied in the period of adoption. + References No definition available. + Details
Name: | us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Details
Name: | us-gaap_AdjustmentsForNewAccountingPronouncementsAxis=fstr_AccountingStandardsUpdate201802Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementEquityComponentsAxis=us-gaap_AccumulatedOtherComprehensiveIncomeMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementEquityComponentsAxis=us-gaap_RetainedEarningsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_AdjustmentsForNewAccountingPronouncementsAxis=us-gaap_AccountingStandardsUpdate201606Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |