Stock-based Compensation Plans - Schedule of Restricted Stock Activity (Detail) - Restricted Stock [Member] - $ / shares	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Units, Beginning balance	158,000	119,000	100,000
Number of Units, Granted	103,000	99,000	71,000
Number of Units, Vested	(69,000)	(55,000)	(48,000)
Number of Units, Forfeited	(19,000)	(5,000)	(4,000)
Number of Units, Ending balance	173,000	158,000	119,000
Weighted Average Grant Date Fair Value, Beginning balance	$ 41.24	$ 41.14	$ 44.16
Weighted Average Grant Date Fair Value, Granted	35.75	42.35	38.83
Weighted Average Grant Date Fair Value, Vested	40.96	42.98	44.07
Weighted Average Grant Date Fair Value, Forfeited	39.31	41.83	40.45
Weighted Average Grant Date Fair Value, Ending balance	$ 38.30	$ 41.24	$ 41.14