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
	Dec. 31, 2016	Dec. 31, 2015
Numerator for diluted and basic earnings per share:
Net income	$ 37,901	$ 19,241
Denominator - share:
Basic weighted-average shares (in shares)	30,989	31,185
Effect of dilutive securities (in shares)	1,547	1,251
Diluted weighted-average shares (in shares)	32,536	32,436
Earnings per share:
Basic (in dollars per share)	$ 1.22	$ 0.62
Diluted (in dollars per share)	$ 1.16	$ 0.59