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.