Supplementary Financial Data (Unaudited) - Selected Unaudited Consolidated Financial Results (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended																12 Months Ended
	Sep. 30, 2020		Jun. 30, 2020		Mar. 31, 2020		Dec. 31, 2019		Sep. 30, 2019		Jun. 30, 2019		Mar. 31, 2019		Dec. 31, 2018		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 374,356		$ 313,731		$ 307,971		$ 298,504		$ 305,344		$ 314,249		$ 278,234		$ 262,256
Cost of revenues	93,676	[1]	88,569	[1]	88,139	[1]	90,758	[1]	87,996	[1]	87,215	[1]	85,568	[1]	76,066	[1]	$ 361,142	$ 336,845	$ 312,898
Gross profit	280,680		225,162		219,832		207,746		217,348		227,034		192,666		186,190
Net income	$ 59,126		$ 64,076		$ 58,288		$ 54,921		$ 54,584		$ 64,152		$ 33,381		$ 40,007		$ 236,411	$ 192,124	$ 126,482
Earnings per share:
Basic earnings per share (in dollars per share)	$ 2.04	[2]	$ 2.21	[2]	$ 2.00	[2]	$ 1.89	[2]	$ 1.89	[2]	$ 2.21	[2]	$ 1.15	[2]	$ 1.38	[2]	$ 8.13	$ 6.63	$ 4.26
Diluted earnings per share (in dollars per share)	$ 1.98	[2]	$ 2.15	[2]	$ 1.94	[2]	$ 1.82	[2]	$ 1.80	[2]	$ 2.12	[2]	$ 1.10	[2]	$ 1.32	[2]	$ 7.90	$ 6.34	$ 4.06
Shares used in computing earnings per share:
Basic weighted-average shares (in shares)	29,045		29,005		29,194		29,025		28,918		28,967		29,074		28,961		29,067	28,980	29,711
Diluted (in shares)	29,833		29,744		29,985		30,169		30,290		30,292		30,259		30,336		29,932	30,294	31,180

[1] Cost of revenues excludes amortization expense of $0.3 million, $0.4 million, $0.5 million, $0.6 million, $0.5 million, $0.5 million, $0.5 million and $0.5 million for the quarters ended September 30, 2020, June 30, 2020, March 31, 2020, December 31, 2019, September 30, 2019, June 30, 2019, March 31, 2019 and December 31, 2018, 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.