Stock-Based Employee Benefit Plans - Summary of Assumptions Used to Estimate Fair Value of Market Stock Units (Details) - Market stock units (MSUs)	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility in FICO’s stock price	36.20%	36.30%	47.30%
Expected volatility in Russell 3000 Index		18.20%	26.00%
Expected volatility in S&P 500 Index (peer average)	30.90%
Correlation between FICO and the Russell 3000 Index		60.30%	73.50%
Correlation between FICO and the S&P 500 Index	30.30%
Risk-free interest rate	4.09%	4.39%	4.02%
Average expected dividend yield	0.00%	0.00%	0.00%