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Stock-Based Employee Benefit Plans - Assumptions Used to Estimate Fair Value of Market Stock Units (Details) - Market stock units (MSUs)
12 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected volatility in FICO’s stock price 41.30% 25.20% 24.60%
Expected volatility in Russell 3000 Index 23.70% 12.90% 12.80%
Correlation between FICO and the Russell 3000 Index 77.50% 64.00% 66.60%
Risk-free interest rate 0.20% 1.67% 2.73%
Average expected dividend yield 0.00% 0.00% 0.00%