| Document and Entity Information | Mar. 30, 2020 |
| Cover page. | |
| Document Type | 8-K |
| Document Period End Date | Mar. 30, 2020 |
| Entity Registrant Name | FASTENAL CO |
| Entity Incorporation, State or Country Code | MN |
| Entity File Number | 0-16125 |
| 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 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common stock, par value $.01 per share |
| Trading Symbol | FAST |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0000815556 |
| Amendment Flag | false |
