Segment Reporting - Reportable Segment ROA Assets (Details) - USD ($) $ in Millions	Sep. 30, 2026	Dec. 31, 2025
U.S. reportable segment
Segment Reporting Information [Line Items]
U.S. ROA assets	$ 2,745.9	$ 2,446.8

Schedule of Segment Reporting Information by Segment [Line Items]

No definition available.

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