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Business Segments - Narratives (Details)	Mar. 31, 2018
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