Supplementary Financial Data (Unaudited) - Selected Unaudited Consolidated Financial Results (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
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 235,824		$ 238,778		$ 206,678		$ 200,076		$ 232,756		$ 209,366		$ 207,109		$ 189,550		$ 881,356		$ 838,781		$ 788,985
Cost of revenues	74,298	[1]	66,384	[1]	62,298	[1]	62,193	[1]	67,042	[1]	66,202	[1]	70,991	[1]	66,300	[1]	265,173	[2]	270,535	[2]	249,281	[2]
Gross profit	161,526		172,394		144,380		137,883		165,714		143,164		136,118		123,250
Net income	$ 32,104		$ 34,987		$ 23,116		$ 19,241		$ 33,319		$ 19,906		$ 18,870		$ 14,407		$ 109,448		$ 86,502		$ 94,879
Earnings per share:
Basic (in dollars per share)	$ 1.04	[3]	$ 1.12	[3]	$ 0.74	[3]	$ 0.62	[3]	$ 1.07	[3]	$ 0.64	[3]	$ 0.60	[3]	$ 0.45	[3]	$ 3.52		$ 2.75		$ 2.80
Diluted (in dollars per share)	$ 1.00	[3]	$ 1.08	[3]	$ 0.72	[3]	$ 0.59	[3]	$ 1.03	[3]	$ 0.62	[3]	$ 0.58	[3]	$ 0.43	[3]	$ 3.39		$ 2.65		$ 2.72
Shares used in computing earnings per 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
Diluted (in shares)	32,221		32,313		32,262		32,436		32,494		32,363		32,448		33,128		32,308		32,609		34,864

[1] Cost of revenues excludes amortization expense of $1.7 million, $1.9 million, $1.9 million, $1.9 million, $1.8 million, $1.9 million, $1.9 million and $1.8 million for the quarters ended September 30, 2016, June 30, 2016, March 31, 2016, December 31, 2015, September 30, 2015, June 30, 2015, March 31, 2015 and December 31, 2014, respectively.

[2] Cost of revenues and selling, general and administrative expenses exclude the amortization of intangible assets. See Note 7.

[3] 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.