Stock-based Compensation Plans - Schedule of Restricted Stock Activity (Detail) - Restricted Stock [Member] - $ / shares	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Units, Beginning balance	100,000	90,000	85,000
Number of Units, Granted	71,000	67,000	45,000
Number of Units, Vested	(48,000)	(43,000)	(39,000)
Number of Units, Forfeited	(4,000)	(14,000)	(1,000)
Number of Units, Ending balance	119,000	100,000	90,000
Weighted Average Grant Date Fair Value, Beginning balance	$ 44.16	$ 43.69	$ 30.30
Weighted Average Grant Date Fair Value, Granted	38.83	41.40	56.05
Weighted Average Grant Date Fair Value, Vested	44.07	38.74	28.61
Weighted Average Grant Date Fair Value, Forfeited	40.45	44.76	47.86
Weighted Average Grant Date Fair Value, Ending balance	$ 41.14	$ 44.16	$ 43.69