Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2015	Jul. 13, 2015
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2015
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	Q2
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		290,165,109