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																12 Months Ended
	Sep. 30, 2018		Jun. 30, 2018		Mar. 31, 2018		Dec. 31, 2017		Sep. 30, 2017		Jun. 30, 2017		Mar. 31, 2017		Dec. 31, 2016		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Numerator for basic and diluted earnings per share
Net income	$ 50,480		$ 32,361		$ 32,275		$ 27,299		$ 40,044		$ 25,227		$ 25,084		$ 37,901		$ 142,415	$ 128,256	$ 109,448
Denominator — share:
Basic weighted-average shares (in shares)	29,077		29,708		29,985		30,078		30,534		30,914		31,017		30,989		29,711	30,862	31,129
Effect of dilutive securities (in shares)																	1,469	1,383	1,179
Diluted weighted-average shares (in shares)	30,702		31,161		31,300		31,561		31,963		32,224		32,260		32,536		31,180	32,245	32,308
Earnings per share:
Basic (in dollars per share)	$ 1.74	[1]	$ 1.09	[1]	$ 1.08	[1]	$ 0.91	[1]	$ 1.31	[1]	$ 0.82	[1]	$ 0.81	[1]	$ 1.22	[1]	$ 4.79	$ 4.16	$ 3.52
Diluted (in dollars per share)	$ 1.64	[1]	$ 1.04	[1]	$ 1.03	[1]	$ 0.86	[1]	$ 1.25	[1]	$ 0.78	[1]	$ 0.78	[1]	$ 1.16	[1]	$ 4.57	$ 3.98	$ 3.39

[1] Earnings per share is computed independently for each of the quarters presented. Therefore, the sum of the quarterly per share amounts may not equal the totals for the respective years.