Consolidated Statements Of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Statement of Comprehensive Income [Abstract]
Net income	$ 69,872	$ 85,905	$ 229,778	$ 332,897
Other comprehensive income:
Foreign currency translation adjustment	184	118	(768)	61
Available-for-sale securities:
Unrealized gains on securities, net of taxes of $490 and $197 for the three months ended June 30, 2020 and 2019, respectively, and $146 and $940 for the nine months ended June 30, 2020 and 2019, respectively	4,061	918	1,230	3,811
Reclassification adjustment for realized losses included in net income, net of taxes of $(47) and $(35) for the three months ended June 30, 2020 and 2019, respectively, and $(65) and $(35) for the nine months ended June 30, 2020 and 2019, respectively	163	112	233	113
Net change in unrealized gains on available-for-sale securities, net of tax	4,224	1,030	1,463	3,924
Total other comprehensive income	4,408	1,148	695	3,985
Comprehensive income	$ 74,280	$ 87,053	$ 230,473	$ 336,882