Income Taxes - Reconciliation of Federal Statutory Tax Rate to Company's Effective Tax Rate (Detail)	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Income Tax Disclosure [Abstract]
Federal statutory tax rate	21.00%	21.00%	24.50%	35.00%
New U.S. tax legislation (see discussion above)	0	(0.043)	0.546
State and local income taxes, net of federal tax benefit	(1.90%)	0.10%	0.80%
Effective Income Tax Rate Reconciliation, Foreign Income Tax Rate Differential, Percent	(14.80%)	(6.60%)	(11.70%)
Effect of foreign operations	19.10%	(5.50%)	19.00%
Effect of Research Credits and FDII/Domestic Production Activities	(5.00%)	(3.30%)	(2.80%)
Effect of tax benefit relating to share-based compensation	(4.50%)	(3.90%)	(6.10%)
Effective Income Tax Rate Reconciliation, Disposition of Asset, Percent	(4.50%)	(2.00%)	1.30%
Effective Income Tax Rate Reconciliation, Uncertain Tax Position, Percent	0.00%	0.00%	3.30%
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Percent	0.00%	0.00%	(4.80%)
Effective Income Tax Rate Reconciliation, Change in Accounting Method, Percent	0.00%	0.00%	(4.50%)
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Share-based Payment Arrangement, Percent	0.00%	0.00%	1.60%
Other, net	1.90%	(0.30%)	(1.70%)
Total	11.30%	(4.80%)	73.50%