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Revenue - Additional Information (Details) | 12 Months Ended
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Dec. 31, 2020
Revenue benchmark | Customer concentration risk |
Concentration Risk [Line Items] |
Percentage of consolidated net sales, single customer | 5.00%

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- Definition Concentration Risk, Threshold Percentage + References No definition available. + Details
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Namespace Prefix: | fast_
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_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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

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- Details
Name: | us-gaap_ConcentrationRiskByTypeAxis=us-gaap_CustomerConcentrationRiskMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |