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, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Numerator for diluted and basic earnings per share:
Net income	$ 58,288	$ 33,381	$ 113,209	$ 73,388
Denominator - share:
Basic weighted-average shares (in shares)	29,194	29,074	29,109	29,017
Effect of dilutive securities (in shares)	791	1,185	967	1,280
Diluted weighted-average shares (in shares)	29,985	30,259	30,076	30,297
Earnings per share:
Basic (in dollars per share)	$ 2.00	$ 1.15	$ 3.89	$ 2.53
Diluted (in dollars per share)	$ 1.94	$ 1.10	$ 3.76	$ 2.42