Postretirement Benefits - Fair Value of Pension Assets by Asset Category and by Level (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 7,078	$ 2,225	$ 4,532
SWITZERLAND
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6,754	1,886
TAIWAN, PROVINCE OF CHINA
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	324	339
Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	7,078	2,225
Significant Other Observable Inputs (Level 2) | SWITZERLAND
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6,754	1,886
Significant Other Observable Inputs (Level 2) | TAIWAN, PROVINCE OF CHINA
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 324	$ 339