Postretirement Benefits - Fair Value of Pension Assets by Asset Category and by Level (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 7,500	$ 6,574	$ 7,078
SWITZERLAND
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	7,371	6,486
TAIWAN, PROVINCE OF CHINA
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	129	88
Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	7,500	6,574
Significant Other Observable Inputs (Level 2) | SWITZERLAND
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	7,371	6,486
Significant Other Observable Inputs (Level 2) | TAIWAN, PROVINCE OF CHINA
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 129	$ 88