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, 2024	Mar. 31, 2023	Mar. 31, 2024	Mar. 31, 2023
Numerator for diluted and basic earnings per share:
Net income	$ 129,799	$ 101,550	$ 250,864	$ 199,193
Denominator — share:
Basic weighted-average shares (in shares)	24,819	25,116	24,791	25,080
Effect of dilutive securities (in shares)	335	303	395	351
Diluted weighted-average shares (in shares)	25,154	25,419	25,186	25,431
Earnings per share:
Basic (in dollars per share)	$ 5.23	$ 4.04	$ 10.12	$ 7.94
Diluted (in dollars per share)	$ 5.16	$ 4.00	$ 9.96	$ 7.83