Stock-based Compensation Plans - Schedule of Restricted Stock Activity (Detail) - Restricted Stock Awards [Member] - $ / shares	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Units, Beginning balance	90,000	85,000	68,000
Number of Units, Granted	67,000	45,000	56,000
Number of Units, Vested	(43,000)	(39,000)	(29,000)
Number of Units, Forfeited	(14,000)	(1,000)	(10,000)
Number of Units, Ending balance	100,000	90,000	85,000
Weighted Average Grant Date Fair Value, Beginning balance	$ 43.69	$ 30.30	$ 23.98
Weighted Average Grant Date Fair Value, Granted	41.40	56.05	34.38
Weighted Average Grant Date Fair Value, Vested	38.74	28.61	23.78
Weighted Average Grant Date Fair Value, Forfeited	44.76	47.86	29.10
Weighted Average Grant Date Fair Value, Ending balance	$ 44.16	$ 43.69	$ 30.30