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, 2018		Jun. 30, 2018		Mar. 31, 2018		Dec. 31, 2017		Sep. 30, 2017		Jun. 30, 2017		Mar. 31, 2017		Dec. 31, 2016		Sep. 30, 2018		Sep. 30, 2017		Sep. 30, 2016
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 279,791		$ 259,505		$ 257,858		$ 235,321		$ 253,205		$ 230,986		$ 228,378		$ 219,600		$ 1,032,475		$ 932,169		$ 881,356
Cost of revenues	79,431	[1]	78,390	[1]	78,519	[1]	74,359	[1]	75,202	[1]	69,793	[1]	72,131	[1]	69,997	[1]	310,699	[2]	287,123	[2]	265,173	[2]
Gross profit	200,360		181,115		179,339		160,962		178,003		161,193		156,247		149,603
Net income	$ 50,480		$ 32,361		$ 32,275		$ 27,299		$ 40,044		$ 25,227		$ 25,084		$ 37,901		$ 142,415		$ 128,256		$ 109,448
Earnings per share:
Basic (in dollars per share)	$ 1.74	[3]	$ 1.09	[3]	$ 1.08	[3]	$ 0.91	[3]	$ 1.31	[3]	$ 0.82	[3]	$ 0.81	[3]	$ 1.22	[3]	$ 4.79		$ 4.16		$ 3.52
Diluted (in dollars per share)	$ 1.64	[3]	$ 1.04	[3]	$ 1.03	[3]	$ 0.86	[3]	$ 1.25	[3]	$ 0.78	[3]	$ 0.78	[3]	$ 1.16	[3]	$ 4.57		$ 3.98		$ 3.39
Shares used in computing earnings per share:
Basic weighted-average shares (in shares)	29,077		29,708		29,985		30,078		30,534		30,914		31,017		30,989		29,711		30,862		31,129
Diluted (in shares)	30,702		31,161		31,300		31,561		31,963		32,224		32,260		32,536		31,180		32,245		32,308

[1] Cost of revenues excludes amortization expense of $0.5 million, $0.6 million, $0.6 million, $0.7 million, $1.4 million, $1.7 million, $1.7 million and $1.7 million for the quarters ended September 30, 2018, June 30, 2018, March 31, 2018, December 31, 2017, September 30, 2017, June 30, 2017, March 31, 2017 and December 31, 2016, 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.