| Document and Entity Information Document | Nov. 05, 2019 |
| Cover page. | |
| Document Type | 8-K |
| Document Period End Date | Nov. 5, 2019 |
| Entity Registrant Name | FIVE9, INC. |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-36383 |
| Entity Central Index Key | 0001288847 |
| Amendment Flag | false |
| Entity Tax Identification Number | 94-3394123 |
| Entity Address, Address Line One | Bishop Ranch 8 |
| Entity Address, Address Line Two | 4000 Executive Parkway, |
| Entity Address, Address Line Three | Suite 400 |
| Entity Address, City or Town | San Ramon, |
| Entity Address, State or Province | CA |
| Entity Address, Postal Zip Code | 94583 |
| City Area Code | 925 |
| Local Phone Number | 201-2000 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Title of 12(b) Security | Common stock, par value $0.001 per share |
| Trading Symbol | FIVN |
| Security Exchange Name | NASDAQ |
