Document and Entity Information - USD ($)	12 Months Ended
	Sep. 30, 2018	Oct. 26, 2018	Mar. 31, 2018
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Sep. 30, 2018
Document Fiscal Year Focus	2018
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		28,955,028
Entity Small Business	false
Entity Public Float			$ 3,548,843,145
Entity Emerging Growth Company	false
Entity Shell Company	false