Income Taxes - Reconciliation of Federal Statutory Tax Rate to Company's Effective Tax Rate (Detail)	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Income Tax Disclosure [Abstract]
Federal statutory tax rate	21.00%	21.00%	21.00%	35.00%
U.S. tax legislation (see discussion above)	0	0	(0.043)
State and local income taxes, net of federal tax benefit	(1.90%)	(1.90%)	0.10%
Effective Income Tax Rate Reconciliation, Foreign Income Tax Rate Differential, Percent	(8.10%)	(14.80%)	(6.60%)
Effect of foreign operations	(0.10%)	19.10%	(5.50%)
Effect of Research Credits and FDII/Domestic Production Activities	(1.60%)	(5.00%)	(3.30%)
Effect of share-based compensation	0.10%	(4.50%)	(3.90%)
Effective Income Tax Rate Reconciliation, Disposition of Asset, Percent	0.00%	(4.50%)	(2.00%)
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Percent	(1.70%)	0.00%	0.00%
Other, net	(1.00%)	1.90%	(0.30%)
Total	6.70%	11.30%	(4.80%)