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, 2016		Jun. 30, 2016		Mar. 31, 2016		Dec. 31, 2015		Sep. 30, 2015		Jun. 30, 2015		Mar. 31, 2015		Dec. 31, 2014		Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Numerator for basic and diluted earnings per share
Net income	$ 32,104		$ 34,987		$ 23,116		$ 19,241		$ 33,319		$ 19,906		$ 18,870		$ 14,407		$ 109,448	$ 86,502	$ 94,879
Denominator — share:
Basic weighted-average shares (in shares)	30,916		31,149		31,268		31,185		31,214		31,118		31,335		31,936		31,129	31,402	33,870
Effect of dilutive securities (in shares)																	1,179	1,207	994
Diluted weighted-average shares (in shares)	32,221		32,313		32,262		32,436		32,494		32,363		32,448		33,128		32,308	32,609	34,864
Earnings per share:
Basic (in dollars per share)	$ 1.04	[1]	$ 1.12	[1]	$ 0.74	[1]	$ 0.62	[1]	$ 1.07	[1]	$ 0.64	[1]	$ 0.60	[1]	$ 0.45	[1]	$ 3.52	$ 2.75	$ 2.80
Diluted (in dollars per share)	$ 1.00	[1]	$ 1.08	[1]	$ 0.72	[1]	$ 0.59	[1]	$ 1.03	[1]	$ 0.62	[1]	$ 0.58	[1]	$ 0.43	[1]	$ 3.39	$ 2.65	$ 2.72

[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.