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Revenue - Over Time Sales (Details) - USD ($) $ in Thousands | 3 Months Ended
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Mar. 31, 2023 | Mar. 31, 2022
Disaggregation of Revenue |  |
Over time sales, amount | $ 32,526 | $ 30,303
Over time sales, percentage | 28.20% | 30.70%
Over time input method |  |
Disaggregation of Revenue |  |
Over time sales, amount | $ 16,211 | $ 19,322
Over time sales, percentage | 14.00% | 19.60%
Over time output method |  |
Disaggregation of Revenue |  |
Over time sales, amount | $ 16,315 | $ 10,981
Over time sales, percentage | 14.10% | 11.10%

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- Definition Over Time Sales, Amount + References No definition available. + Details
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- Definition Over Time Sales, Percent + References No definition available. + Details
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Namespace Prefix: | fstr_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | duration

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- 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
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Namespace Prefix: | us-gaap_
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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Data Type: | na
Balance Type: |
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