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Segment Reporting - Reportable Segment ROA Assets (Details) - USD ($) $ in Millions	Sep. 30, 2025	Dec. 31, 2024
U.S. reportable segment
Segment Reporting Information [Line Items]
U.S. ROA assets	$ 2,531.6	$ 2,189.6

X

- Definition

Schedule of Segment Reporting Information by Segment [Line Items]

+ References

No definition available.

+ Details

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Namespace Prefix:	fast_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Segment Reporting, Segment Item, ROA Assets

+ References

No definition available.

+ Details

Name:	fast_SegmentReportingSegmentItemROAAssets
Namespace Prefix:	fast_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=us-gaap_ReportableSegmentAggregationBeforeOtherOperatingSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: