Supplementary Financial Data - Selected Unaudited Consolidated Financial Results (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
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 232,756		$ 209,366		$ 207,109		$ 189,550		$ 221,570		$ 197,610		$ 185,462		$ 184,343		$ 838,781		$ 788,985		$ 743,444
Cost of revenues	67,042	[1]	66,202	[1]	70,991	[1]	66,300	[1]	71,026	[2]	62,752	[2]	58,183	[2]	57,319	[2]	270,535	[3]	249,281	[3]	229,468	[3]
Gross profit	165,714		143,164		136,118		123,250		150,544		134,858		127,279		127,024
Net income	$ 33,319		$ 19,906		$ 18,870		$ 14,407		$ 36,603		$ 20,548		$ 20,751		$ 16,977		$ 86,502		$ 94,879		$ 90,095
Earnings per share:
Basic (in dollars per share)	$ 1.07	[2]	$ 0.64	[2]	$ 0.60	[2]	$ 0.45	[2]	$ 1.14	[1]	$ 0.60	[1]	$ 0.60	[1]	$ 0.49	[1]	$ 2.75		$ 2.80		$ 2.55
Diluted (in dollars per share)	$ 1.03	[2]	$ 0.62	[2]	$ 0.58	[2]	$ 0.43	[2]	$ 1.10	[1]	$ 0.58	[1]	$ 0.59	[1]	$ 0.47	[1]	$ 2.65		$ 2.72		$ 2.48
Shares used in computing earnings per 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
Diluted (shares)	32,494		32,363		32,448		33,128		33,217		35,162		35,311		35,820		32,609		34,864		36,292

[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.

[2] 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.

[3] Cost of revenues and selling, general and administrative expenses exclude the amortization of intangible assets. See Note 7 to the consolidated financial statements.