Consolidated Statements Of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2017	Mar. 31, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 93,138	$ 75,415	$ 187,356	$ 165,131
Other comprehensive gain (loss):
Foreign currency translation adjustment	331	1,687	(1,748)	1,591
Unrealized gains (losses) on securities, net of taxes of $163 and $760 for the three months ended March 31, 2017 and 2016, respectively, and $(301) and $74 for the six months ended March 31, 2017 and 2016, respectively	271	1,266	(502)	123
Reclassification adjustment for realized (gains) losses included in net income, net of taxes of $14 and $1 for the three months ended March 31, 2017 and 2016, respectively, and $15 and $(9) for the six months ended March 31, 2017 and 2016, respectively	(23)	(2)	(25)	15
Net change in unrealized gains (losses) on available-for-sale securities, net of tax	248	1,264	(527)	138
Total other comprehensive gain (loss)	579	2,951	(2,275)	1,729
Comprehensive income	$ 93,717	$ 78,366	$ 185,081	$ 166,860