Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Income Taxes [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent		24.50%	35.00%	35.00%
Foreign pretax earnings		$ 13,000	$ 27,800	$ 33,000
Current foreign tax expense related to foreign tax withholdings		6,000	4,600	6,500
Unrecognized tax benefits		6,113	$ 6,480	$ 6,799	$ 4,634
Unrecognized tax benefits that would impact the effective tax rate if recognized		5,800
Unrecognized tax benefits, accrued interest		300
Unremitted earnings of foreign subsidiaries		79,700
Deemed Repatriation of Earnings of Foreign Subsidiaries		57,900
Tax Cuts and Jobs Act of 2017, Income Tax Expense, Non-deductibility of Performance-based Compensation, Provisional Amount		6,000
Tax Cuts and Jobs Act of 2017, Transition Tax for Accumulated Foreign Earnings, Income Tax Expense (Benefit), Incomplete Accounting, Provisional Amount		3,600
Tax Cuts and Jobs Act of 2017, Income Tax Expense (Benefit), Incomplete Accounting for Tax Effects, Provisional Amount		9,600
Tax Cuts and Jobs Act of 2017, Change in Tax Rate, Deferred Tax Asset and Liability, Income Tax (Expense) Benefit		14,000
California Franchise Tax Board | Research Tax Credit Carryforward
Income Taxes [Line Items]
Excess state research credit		10,500
Valuation allowance		10,500
U.S. Federal
Income Taxes [Line Items]
Net operating loss carryforwards		$ 13,700
Net operating loss carryforwards expiration dates		2020
State
Income Taxes [Line Items]
Net operating loss carryforwards		$ 200
Net operating loss carryforwards expiration dates		2021
Foreign NOL carryforwards
Income Taxes [Line Items]
Net operating loss carryforwards		$ 41,900
Foreign NOL carryforwards | China
Income Taxes [Line Items]
Net operating loss carryforwards		$ 25,100
Scenario, Forecast [Member]
Income Taxes [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	21.00%