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, 2017	Mar. 31, 2016	Mar. 31, 2017	Mar. 31, 2016
Numerator for diluted and basic earnings per share:
Net income	$ 25,084	$ 23,116	$ 62,985	$ 42,357
Denominator - share:
Basic weighted-average shares (in shares)	31,017	31,268	31,003	31,226
Effect of dilutive securities (in shares)	1,243	994	1,395	1,123
Diluted weighted-average shares (in shares)	32,260	32,262	32,398	32,349
Earnings per share:
Basic (in dollars per share)	$ 0.81	$ 0.74	$ 2.03	$ 1.36
Diluted (in dollars per share)	$ 0.78	$ 0.72	$ 1.94	$ 1.31