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