Document And Entity Information (USD $)	12 Months Ended
	Dec. 31, 2014	Jan. 23, 2015	Jun. 30, 2014
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2014
Document Fiscal Year Focus	2014
Document Fiscal Period Focus	FY
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		295,880,219
Entity Public Float			$ 14,587,577,033
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Well-known Seasoned Issuer	Yes
Trading Symbol	fast