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		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Numerator for diluted and basic earnings per share:
Net income	$ 64,152	$ 29,721	$ 137,540	$ 93,769
Denominator - share:
Basic weighted-average shares (in shares)	28,967	29,708	29,000	29,924
Effect of dilutive securities (in shares)	1,325	1,453	1,295	1,417
Diluted weighted-average shares (in shares)	30,292	31,161	30,295	31,341
Earnings per share:
Basic (in dollars per share)	$ 2.21	$ 1.00	$ 4.74	$ 3.13
Diluted (in dollars per share)	$ 2.12	$ 0.95	$ 4.54	$ 2.99