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Cover - USD ($)
 12 Months Ended
Sep. 30, 2021
Oct. 29, 2021
Mar. 31, 2021
Cover [Abstract]      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Sep. 30, 2021    
Document Transition Report false    
Entity File Number 1-11689    
Entity Registrant Name Fair Isaac Corp    
Entity Incorporation, State or Country Code DE    
Entity Tax Identification Number 94-1499887    
Entity Address, Address Line One 5 West Mendenhall, Suite 105    
Entity Address, City or Town Bozeman,    
Entity Address, State or Province MT    
Entity Address, Postal Zip Code 59715    
City Area Code 406    
Local Phone Number -7276    
Title of 12(b) Security Common Stock, $0.01 par value per share    
Trading Symbol FICO    
Security Exchange Name NYSE    
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Large Accelerated Filer    
Entity Small Business false    
Entity Emerging Growth Company false    
ICFR Auditor Attestation Flag true    
Entity Shell Company false    
Entity Public Float     $ 11,138,645,215
Entity Common Stock, Shares Outstanding   27,358,353  
Amendment Flag false    
Document Fiscal Year Focus 2021    
Document Fiscal Period Focus FY    
Entity Central Index Key 0000814547    
Current Fiscal Year End Date --09-30