Postretirement Benefits - Net Periodic Pension Cost (Detail) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Compensation and Retirement Disclosure [Abstract]
Service cost	$ 548	$ 482	$ 406
Interest cost	71	124	154
Expected return on plan assets	(159)	(210)	(214)
Amortization of losses	2	2	2
Other	0	0	0
Net periodic pension cost	462	398	348
Curtailment gain	(227)	0	0
Settlement loss	0	232	0
Total pension cost	$ 235	$ 630	$ 348