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, 2023	Mar. 31, 2022	Mar. 31, 2023	Mar. 31, 2022
Numerator for diluted and basic earnings per share:
Net income	$ 101,550	$ 104,383	$ 199,193	$ 189,342
Denominator — share:
Basic weighted-average shares (in shares)	25,116	26,145	25,080	26,662
Effect of dilutive securities (in shares)	303	276	351	316
Diluted weighted-average shares (in shares)	25,419	26,421	25,431	26,978
Earnings per share:
Basic (in dollars per share)	$ 4.04	$ 3.99	$ 7.94	$ 7.10
Diluted (in dollars per share)	$ 4.00	$ 3.95	$ 7.83	$ 7.02