Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2018	Apr. 10, 2018
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	Q1
Trading Symbol	fast
Entity Registrant Name	FASTENAL CO
Entity Central Index Key	0000815556
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		287,688,839