Stock-based Compensation Plans - Schedule of Restricted Stock Activity (Detail) - Restricted Stock [Member] - $ / shares	12 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Units, Beginning balance	119,000	100,000	90,000
Number of Units, Granted	99,000	71,000	67,000
Number of Units, Vested	(55,000)	(48,000)	(43,000)
Number of Units, Forfeited	(5,000)	(4,000)	(14,000)
Number of Units, Ending balance	158,000	119,000	100,000
Weighted Average Grant Date Fair Value, Beginning balance	$ 41.14	$ 44.16	$ 43.69
Weighted Average Grant Date Fair Value, Granted	42.35	38.83	41.40
Weighted Average Grant Date Fair Value, Vested	42.98	44.07	38.74
Weighted Average Grant Date Fair Value, Forfeited	41.83	40.45	44.76
Weighted Average Grant Date Fair Value, Ending balance	$ 41.24	$ 41.14	$ 44.16