Retirement Plans (Narrative) (Details) $ in Thousands	12 Months Ended
Dec. 31, 2017 USD ($) plan
Defined Benefit Plan Disclosure
Net loss to be recognize during the next fiscal period \| $	$ 96
Pension Plan
Defined Benefit Plan Disclosure
Number of defined contribution plans	6
Pension Plan \| UNITED STATES
Defined Benefit Plan Disclosure
Number of retirement plans	3
Number of defined benefit plans	1
Number of defined contribution plans	2
Pension Plan \| CANADA
Defined Benefit Plan Disclosure
Number of defined contribution plans	2
Pension Plan \| UNITED KINGDOM
Defined Benefit Plan Disclosure
Number of defined benefit plans	1
Number of defined contribution plans	2
Expected future contribution by employer during next fiscal period \| $	$ 253

Number of defined benefit plans offered by the company.

No definition available.

Name:	fstr_DefinedBenefitPlanNumber
Namespace Prefix:	fstr_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of defined contribution plans offered by the company.

+ References

No definition available.

+ Details

Name:	fstr_DefinedContributionPlanNumber
Namespace Prefix:	fstr_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of retirement plans offered by the company.

+ References

No definition available.

+ Details

Name:	fstr_NumberOfRetirementPlans
Namespace Prefix:	fstr_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_DefinedBenefitPlanDisclosureLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of cost (credit) included in accumulated other comprehensive (income) loss expected to be recognized in net periodic benefit cost (credit) for fiscal year following most recent annual statement of financial position.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (s)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

+ Details

Name:	us-gaap_DefinedBenefitPlanExpectedAmortizationNextFiscalYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of contributions expected to be received by defined benefit plan from employer in next fiscal year following latest fiscal year.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

+ Details

Name:	us-gaap_DefinedBenefitPlanExpectedFutureEmployerContributionsNextFiscalYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_RetirementPlanTypeAxis=us-gaap_PensionPlansDefinedBenefitMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RetirementPlanSponsorLocationAxis=country_US
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RetirementPlanSponsorLocationAxis=country_CA
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RetirementPlanSponsorLocationAxis=country_GB
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: