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Business Segments - Narratives (Details) | Mar. 31, 2018
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Segment Reporting [Abstract] |
Monthly allocation of corporate expenses | 1.00%

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- Definition Monthly Allocation Corporate Operating Expense + References No definition available. + Details
Name: | fstr_MonthlyAllocationCorporateOperatingExpense
Namespace Prefix: | fstr_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- References No definition available. + Details
Name: | us-gaap_SegmentReportingAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration