Selected Unaudited Consolidated Financial Results (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended																12 Months Ended
	Sep. 30, 2013		Jun. 30, 2013		Mar. 31, 2013		Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2011		Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2011
Quarterly Financial Information [Line Items]
Revenues	$ 190,327		$ 183,772		$ 179,325		$ 190,020		$ 186,075		$ 160,478		$ 159,521		$ 170,349		$ 743,444		$ 676,423		$ 619,683
Cost of revenues	56,809	[1]	57,655	[1]	58,856	[1]	56,148	[1]	55,327	[1]	47,832	[1]	48,814	[1]	45,974	[1]	229,468	[2]	197,947	[2]	186,470	[2]
Gross profit	133,518		126,117		120,469		133,872		130,748		112,646		110,707		124,375
Net income	$ 28,557		$ 19,622		$ 18,495		$ 23,421		$ 21,246		$ 20,749		$ 20,012		$ 29,997		$ 90,095		$ 92,004		$ 71,562
Earnings per share :
Basic	$ 0.81	[3]	$ 0.55	[3]	$ 0.52	[3]	$ 0.67	[3]	$ 0.62	[3]	$ 0.61	[3]	$ 0.57	[3]	$ 0.83	[3]	$ 2.55		$ 2.64		$ 1.82
Diluted	$ 0.79	[3]	$ 0.54	[3]	$ 0.51	[3]	$ 0.65	[3]	$ 0.60	[3]	$ 0.59	[3]	$ 0.55	[3]	$ 0.81	[3]	$ 2.48		$ 2.55		$ 1.79
Shares used in computing earnings per share:
Basic	35,132		35,499		35,664		35,043		34,262		34,004		35,331		36,034		35,332		34,909		39,359
Diluted	36,151		36,385		36,492		36,151		35,513		35,293		36,552		36,887		36,292		36,063		39,988

[1] Cost of revenues excludes amortization expense of $1.7 million, $1.7 million, $1.7 million, $1.5 million, $0.5 million, $0.1 million, $0.1 million and $0.6 million for the quarters ended September 30, 2013, June 30, 2013, March 31, 2013, December 31, 2012, September 30, 2012, June 30, 2012, March 31, 2012 and December 31, 2011, respectively.

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

[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.