| Document and Entity Information | Jan. 30, 2020 |
| Cover [Abstract] | |
| Entity Registrant Name | FAIR ISAAC CORP |
| Amendment Flag | false |
| Entity Central Index Key | 0000814547 |
| Document Type | 8-K |
| Document Period End Date | Jan. 30, 2020 |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 1-11689 |
| Entity Tax Identification Number | 94-1499887 |
| Entity Address, Address Line One | 181 Metro Drive |
| Entity Address, Address Line Two | Suite 700 |
| Entity Address, City or Town | San Jose |
| Entity Address, State or Province | CA |
| Entity Address, Postal Zip Code | 95110-1346 |
| City Area Code | 408 |
| Local Phone Number | 535-1500 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common Stock, $0.01 par value per share |
| Trading Symbol | FICO |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | false |
