Label	Element	Value
Accounting Standards Update 2016-06 [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption	us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption	$ (305,000)
Accounting Standards Update 2016-06 [Member] | Retained Earnings [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption	us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption	(305,000)
Accounting Standards Update 2018-02 [Member] | AOCI Attributable to Parent [Member]
Tax Cuts And Jobs Act Of 2017 Reclassification From Aoci To Retained Earnings Tax Effect	fstr_TaxCutsAndJobsActOf2017ReclassificationFromAociToRetainedEarningsTaxEffect	(633,000)
Accounting Standards Update 2018-02 [Member] | Retained Earnings [Member]
Tax Cuts And Jobs Act Of 2017 Reclassification From Aoci To Retained Earnings Tax Effect	fstr_TaxCutsAndJobsActOf2017ReclassificationFromAociToRetainedEarningsTaxEffect	$ 633,000