| Cover Page Document | Jan. 21, 2020 |
| Cover page. | |
| Title of 12(b) Security | Common stock, no par value |
| Entity Central Index Key | 0001048695 |
| City Area Code | 206 |
| Entity Address, Address Line One | 801 5th Avenue |
| Entity Incorporation, State or Country Code | WA |
| Document Type | 8-K |
| Document Period End Date | Jan. 21, 2020 |
| Entity Registrant Name | F5 Networks, Inc. |
| Entity File Number | 000-26041 |
| Entity Tax Identification Number | 91-1714307 |
| Entity Address, City or Town | Seattle |
| Entity Address, State or Province | WA |
| Entity Address, Postal Zip Code | 98104 |
| Local Phone Number | 272-5555 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Amendment Flag | false |
| Entity Emerging Growth Company | false |
| Trading Symbol | FFIV |
| Security Exchange Name | NASDAQ |
| Current Fiscal Year End Date | --09-30 |
