Document and Entity Information - USD ($)
12 Months Ended
Sep. 30, 2019
Oct. 25, 2019
Mar. 31, 2019
Document And Entity Information [Abstract]      
Title of 12(b) Security Common Stock, $0.01 par value per share    
Entity Address, Address Line One 181 Metro Drive, Suite 700    
Entity Incorporation, State or Country Code DE    
Document Transition Report false    
Document Annual Report true    
Document Type 10-K    
Amendment Flag false    
Document Period End Date Sep. 30, 2019    
Document Fiscal Year Focus 2019    
Document Fiscal Period Focus FY    
Trading Symbol FICO    
Entity Registrant Name Fair Isaac Corp    
Entity Central Index Key 0000814547    
Current Fiscal Year End Date --09-30    
Entity Well-known Seasoned Issuer Yes    
Entity Current Reporting Status Yes    
Entity Voluntary Filers No    
Entity Filer Category Large Accelerated Filer    
Entity Common Stock, Shares Outstanding   28,961,612  
Entity Small Business false    
Entity Public Float     $ 5,769,737,106
Entity Emerging Growth Company false    
Entity Shell Company false    
Entity File Number 1-11689    
Entity Tax Identification Number 94-1499887    
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    
Security Exchange Name NYSE    
Entity Interactive Data Current Yes