Earnings per Share - Reconciliation of Numerators and Denominators of Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Numerator for basic and diluted earnings per share
Net income	$ 651,946	$ 512,811	$ 429,375
Denominator — share:
Basic weighted-average shares (in shares)	24,239	24,676	24,986
Effect of dilutive securities (in shares)	322	403	381
Diluted weighted-average shares (in shares)	24,561	25,079	25,367
Basic earnings per share (in dollars per share)	$ 26.90	$ 20.78	$ 17.18
Diluted earnings per share (in dollars per share)	$ 26.54	$ 20.45	$ 16.93