| Document and Entity Information | Nov. 20, 2019 |
| Cover page. | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Nov. 20, 2019 |
| Current Fiscal Year End Date | --12-31 |
| Entity Registrant Name | FORESCOUT TECHNOLOGIES, INC. |
| Entity Central Index Key | 0001145057 |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-38253 |
| Entity Tax Identification Number | 51-0406800 |
| Entity Address, Address Line One | 190 West Tasman Drive |
| Entity Address, City or Town | San Jose |
| Entity Address, State or Province | CA |
| Entity Address, Postal Zip Code | 95134 |
| City Area Code | 408 |
| Local Phone Number | 213-3191 |
| 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 $0.001 per share |
| Trading Symbol | FSCT |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
