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v3.7.0.1

Business Segments (Narrative) (Details) | 12 Months Ended
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Dec. 31, 2016
Segment Reporting [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

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