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, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Numerator for diluted and basic earnings per share:
Net income	$ 33,381	$ 31,169	$ 73,388	$ 64,048
Denominator - share:
Basic weighted-average shares (in shares)	29,074	29,985	29,017	30,032
Effect of dilutive securities (in shares)	1,185	1,315	1,280	1,399
Diluted weighted-average shares (in shares)	30,259	31,300	30,297	31,431
Earnings per share:
Basic (in dollars per share)	$ 1.15	$ 1.04	$ 2.53	$ 2.13
Diluted (in dollars per share)	$ 1.10	$ 1.00	$ 2.42	$ 2.04