Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2020	Apr. 13, 2020
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Mar. 31, 2020
Document Transition Report	false
Entity File Number	0-16125
Entity Registrant Name	FASTENAL CO
Entity Central Index Key	0000815556
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q1
Amendment Flag	false
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 Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		572,818,887