Document and Entity Information - shares
3 Months Ended
Dec. 31, 2018
Jan. 18, 2019
Document And Entity Information [Abstract]    
Entity Registrant Name FAIR ISAAC CORP  
Trading Symbol FICO  
Entity Central Index Key 0000814547  
Current Fiscal Year End Date --09-30  
Entity Filer Category Large Accelerated Filer  
Document Type 10-Q  
Document Period End Date Dec. 31, 2018  
Document Fiscal Year Focus 2019  
Document Fiscal Period Focus Q1  
Amendment Flag false  
Entity Small Business false  
Emerging Growth Company false  
Entity Common Stock, Shares Outstanding   29,071,202