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Business Segments (Narrative) (Details) | 6 Months Ended
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Jun. 30, 2016
Business Segments [Abstract] |
Capital Charges Rate | 1.00%

X
- Definition The internal cost of capital rate charged per month to a segment for assets used by the segment. + References No definition available. + Details
Name: | fstr_CapitalChargesRate
Namespace Prefix: | fstr_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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