| Document And Entity Information - USD ($) | 12 Months Ended | | |
| Dec. 31, 2021 | Jan. 21, 2022 | Jun. 30, 2021 |
| Cover [Abstract] | | | |
| Document Type | 10-K/A | | |
| Document Annual Report | true | | |
| Document Period End Date | Dec. 31,
2021 | | |
| Document Transition Report | false | | |
| Entity File Number | 0-16125 | | |
| Entity Registrant Name | FASTENAL CO | | |
| Entity Incorporation, State or Country Code | MN | | |
| Entity Tax Identification Number | 41-0948415 | | |
| Entity Address, Address Line One | 2001 Theurer Boulevard | | |
| Entity Address, City or Town | Winona | | |
| Entity Address, State or Province | MN | | |
| Entity Address, Postal Zip Code | 55987-1500 | | |
| City Area Code | 507 | | |
| Local Phone Number | 454-5374 | | |
| Title of 12(b) Security | Common stock, par value $.01 per share | | |
| Trading Symbol | FAST | | |
| Security Exchange Name | NASDAQ | | |
| Entity Well-known Seasoned Issuer | Yes | | |
| Entity Voluntary Filers | No | | |
| Entity Current Reporting Status | Yes | | |
| Entity Interactive Data Current | Yes | | |
| Entity Filer Category | Large Accelerated Filer | | |
| Entity Small Business | false | | |
| Entity Emerging Growth Company | false | | |
| ICFR Auditor Attestation Flag | true | | |
| Entity Shell Company | false | | |
| Entity Public Float | | | $ 29,835,146,952 |
| Entity Common Stock, Shares Outstanding (in shares) | | 575,550,072 | |
| Entity Central Index Key | 0000815556 | | |
| Current Fiscal Year End Date | --12-31 | | |
| Document Fiscal Year Focus | 2021 | | |
| Document Fiscal Period Focus | FY | | |
| Amendment Flag | true | | |
| Amendment Description | This Amendment No. 1 on Form 10-K/A (this "Amendment") amends the original report on Form 10-K for the fiscal year ended December 31, 2021 filed by Fastenal Company (the "Company") on February 7, 2022 (the "Original Filing"). The sole purpose of this Amendment is (i) to include the Company's 2021 Annual Report to Shareholders (the "Annual Report") as Exhibit 99.1 in the Exhibit Index, which was omitted from the Exhibit Index to the Original Filing and (ii) to change Exhibit 10.4 to refer to the Company's Non-Employee Director Stock Option Plan, as amended and restated effective December 20, 2021. The Annual Report shall be deemed furnished, not filed, with the Securities and Exchange Commission in its entirety as no portions thereof are expressly incorporated by reference into the Original Filing.Except as described above, this Amendment does not amend, update or change any other items or disclosures contained in the Original Filing, and accordingly, this Amendment does not reflect or purport to reflect any information or events occurring after the original filing date or modify or update those disclosures affected by subsequent events. Accordingly, this Amendment should be read in conjunction with the Original Filing and the Company's other filings with the Securities and Exchange Commission.Pursuant to Rule 12b-15 under the Securities Exchange Act of 1934, this Amendment also includes as exhibits the certifications of the Principal Executive Officer and Principal Financial Officer of the Company pursuant to Section 302 of the Sarbanes-Oxley Act of 2002. Because no financial statements have been included in this Amendment and this Amendment does not contain or amend any disclosure with respect to Items 307 and 308 of Regulation S-K, paragraphs 3, 4, and 5 have been omitted from such certifications. The Company is not including certifications pursuant to Section 1350 of Chapter 63 of Title 18 of the United States Code (18 U.S.C. 1350) as no financial statements are being filed with this Amendment. | | |
