Income Taxes - Reconciliation of the Federal Statutory Tax Rate to the Total Tax Provision (Detail)	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Income Tax Disclosure [Abstract]
Federal income taxes computed at the statutory rate	21.00%	21.00%
State income taxes, net of federal benefit	1.30%	1.60%
Research & development tax refunds & credits	(6.30%)	(1.90%)
Dividend received deduction	(1.20%)	(0.60%)
263A Section 481(a) adjustment	1.50%
Other, net	0.90%	0.40%
Effective income tax rate	17.20%	20.50%