Income Taxes - Reconciliation of Federal Statutory Tax Rate to Company's Effective Tax Rate (Detail)	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Income Tax Disclosure [Abstract]
Federal statutory tax rate	24.50%	35.00%	35.00%
New U.S. tax legislation (see discussion above)	54.60%	0.00%	0.00%
State and local income taxes, net of federal tax benefit	0.80%	(2.60%)	1.50%
Effect of foreign and Puerto Rico earnings and foreign tax credits	7.30%	(40.80%)	(23.70%)
Effect of Research Credits and Domestic Production Activities	(2.80%)	(2.70%)	(4.40%)
Effect of change in accounting for excess tax benefit relating to share-based compensation (see Note 2)	6.70%	7.90%	0.00%
Effective Income Tax Rate Reconciliation, Disposition of Asset, Percent	1.30%	0.00%	0.00%
Effective Income Tax Rate Reconciliation, Uncertain Tax Position, Percent	3.30%	0.00%	0.00%
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Percent	(4.80%)	0.00%	0.00%
Effective Income Tax Rate Reconciliation, Change in Accounting Method, Percent	(4.50%)	0.00%	0.00%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Share-based Compensation Cost, Percent	1.60%	0.00%	0.00%
Other, net	(1.10%)	6.30%	0.70%
Total	73.50%	(12.70%)	9.10%