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
	Dec. 31, 2019	Dec. 31, 2018
Numerator for diluted and basic earnings per share:
Net income	$ 54,921	$ 40,007
Denominator - share:
Basic weighted-average shares (in shares)	29,025	28,961
Effect of dilutive securities (in shares)	1,144	1,375
Diluted weighted-average shares (in shares)	30,169	30,336
Earnings per share:
Basic (in dollars per share)	$ 1.89	$ 1.38
Diluted (in dollars per share)	$ 1.82	$ 1.32