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, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Numerator for diluted and basic earnings per share:
Net income	$ 68,674	$ 58,288	$ 155,166	$ 113,209
Denominator - share:
Basic weighted-average shares (in shares)	29,087	29,194	29,107	29,109
Effect of dilutive securities (in shares)	444	791	553	967
Diluted weighted-average shares (in shares)	29,531	29,985	29,660	30,076
Earnings per share:
Basic (in dollars per share)	$ 2.36	$ 2.00	$ 5.33	$ 3.89
Diluted (in dollars per share)	$ 2.33	$ 1.94	$ 5.23	$ 3.76