Postretirement Benefits - Fair Value of Pension Assets by Asset Category and by Level (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 2,436	$ 4,734	$ 4,838
Swiss Life collective foundation
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,886	4,208
Taiwan collective trust
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	550	526
Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,436	4,734
Significant Other Observable Inputs (Level 2) | Swiss Life collective foundation
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,886	4,208
Significant Other Observable Inputs (Level 2) | Taiwan collective trust
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 550	$ 526