Postretirement Benefits - Fair Value of Pension Assets by Asset Category and by Level (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 6,574	$ 7,078	$ 2,225
SWITZERLAND
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6,486	6,754
TAIWAN, PROVINCE OF CHINA
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	88	324
Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6,574	7,078
Significant Other Observable Inputs (Level 2) | SWITZERLAND
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6,486	6,754
Significant Other Observable Inputs (Level 2) | TAIWAN, PROVINCE OF CHINA
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 88	$ 324