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Income Taxes - Reconciliation of the Federal Statutory Tax Rate to the Total Tax Provision (Detail)
12 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Income Tax Disclosure [Abstract]    
Federal income taxes computed at the statutory rate 21.00% 21.00%
State income taxes, net of federal benefit 1.60% 1.30%
Research & development tax refunds & credits (5.10%) (6.30%)
Dividend received deduction (1.90%) (1.20%)
Other, net (3.10%) 2.40%
Effective income tax rate 12.50% 17.20%