Earnings per Share - Reconciliation of Numerators and Denominators of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Numerator for diluted and basic earnings per share:
Net income	$ 86,492	$ 54,921
Denominator - share:
Basic weighted-average shares (in shares)	29,127	29,025
Effect of dilutive securities (in shares)	662	1,144
Diluted weighted-average shares (in shares)	29,789	30,169
Earnings per share:
Basic (in dollars per share)	$ 2.97	$ 1.89
Diluted (in dollars per share)	$ 2.90	$ 1.82