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		6 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Mar. 31, 2022	Mar. 31, 2021
Numerator for diluted and basic earnings per share:
Net income	$ 104,383	$ 68,674	$ 189,342	$ 155,166
Denominator — share:
Basic weighted-average shares (in shares)	26,145	29,087	26,662	29,107
Effect of dilutive securities (in shares)	276	444	316	553
Diluted weighted-average shares (in shares)	26,421	29,531	26,978	29,660
Earnings per share:
Basic (in dollars per share)	$ 3.99	$ 2.36	$ 7.10	$ 5.33
Diluted (in dollars per share)	$ 3.95	$ 2.33	$ 7.02	$ 5.23