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, 2015		Jun. 30, 2015		Mar. 31, 2015		Dec. 31, 2014		Sep. 30, 2014		Jun. 30, 2014		Mar. 31, 2014		Dec. 31, 2013		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2013
Numerator for basic and diluted earnings per share
Net income	$ 33,319		$ 19,906		$ 18,870		$ 14,407		$ 36,603		$ 20,548		$ 20,751		$ 16,977		$ 86,502	$ 94,879	$ 90,095
Denominator — share:
Basic weighted-average shares	31,214		31,118		31,335		31,936		32,123		34,210		34,500		34,699		31,402	33,870	35,332
Effect of dilutive securities																	1,207	994	960
Diluted weighted-average shares	32,494		32,363		32,448		33,128		33,217		35,162		35,311		35,820		32,609	34,864	36,292
Earnings per share:
Basic (in dollars per share)	$ 1.07	[1]	$ 0.64	[1]	$ 0.60	[1]	$ 0.45	[1]	$ 1.14	[2]	$ 0.60	[2]	$ 0.60	[2]	$ 0.49	[2]	$ 2.75	$ 2.80	$ 2.55
Diluted (in dollars per share)	$ 1.03	[1]	$ 0.62	[1]	$ 0.58	[1]	$ 0.43	[1]	$ 1.10	[2]	$ 0.58	[2]	$ 0.59	[2]	$ 0.47	[2]	$ 2.65	$ 2.72	$ 2.48

[1] Cost of revenues excludes amortization expense of $1.9 million, $1.9 million, $1.9 million, $1.8 million, $1.9 million, $1.9 million, $1.8 million and $1.8 million for the quarters ended September 30, 2015, June 30, 2015, March 31, 2015, December 31, 2014, September 30, 2014, June 30, 2014, March 31, 2014 and December 31, 2013, respectively.

[2] 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.