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, 2018	Dec. 31, 2017
Numerator for diluted and basic earnings per share:
Net income	$ 40,007	$ 32,879
Denominator - share:
Basic weighted-average shares (in shares)	28,961	30,078
Effect of dilutive securities (in shares)	1,375	1,483
Diluted weighted-average shares (in shares)	30,336	31,561
Earnings per share:
Basic (in dollars per share)	$ 1.38	$ 1.09
Diluted (in dollars per share)	$ 1.32	$ 1.04