Consolidated Statements of Comprehensive Income Statement - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Statement of Comprehensive Income [Abstract]
Net income	$ 307,441	$ 427,734	$ 453,689
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Foreign currency translation adjustment	(572)	(837)	(1,415)
Available-for-sale securities:
Unrealized gains (losses) on securities, net of taxes of $76, $954, and $(869) for the years ended September 30, 2020, 2019, and 2018, respectively	809	3,715	(2,775)
Reclassification adjustment for realized losses included in net income, net of taxes of $(65), $(35), and $(4) for the years ended September 30, 2020, 2019, and 2018, respectively	237	110	9
Net change in unrealized gains (losses) on available-for-sale securities, net of tax	1,046	3,825	(2,766)
Total other comprehensive income (loss)	474	2,988	(4,181)
Comprehensive income	$ 307,915	$ 430,722	$ 449,508