Summary of Significant Accounting Policies (Details)	Dec. 31, 2025	Dec. 31, 2024
Fastenal Company common stock \| Fastenal Company & Subsidiaries 401(k) and Employee Stock Ownership Plan
Accounting Policy [Line Items]
Risk and Uncertainty, Concentration of Investment at Fair Value to Total Investments, Percentage	26.00%	27.00%

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.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 962
-SubTopic 205
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6A-02)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477804/962-205-S99-1

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 962
-SubTopic 235
-Name Accounting Standards Codification
-Section S50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479143/962-235-S50-1

Name:	us-gaap-ebp_EmployeeBenefitPlanAccountingPolicyLineItems
Namespace Prefix:	us-gaap-ebp_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration