Income Taxes - Reconciliation of the Federal Statutory Tax Rate to the Total Tax Provision (Detail)	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Income Tax Disclosure [Abstract]
Federal income taxes computed at the statutory rate	34.00%	34.00%	34.00%
State income taxes, net of federal benefit	1.20%	1.50%	3.30%
Research & development tax refunds & credits	(2.10%)	(2.80%)	5.20%
Dividend received deduction	(0.90%)	(2.20%)
Domestic production activities deduction	(2.80%)	(1.90%)
Domestic international sales corporation benefits			5.80%
Other, net	1.50%	(3.50%)	0.40%
Effective income tax rate	30.90%	25.10%	48.70%