Summary of Significant Accounting Policies - ASC 606 - Impact - Statement of Operations (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue	$ 780,848	$ 631,560	$ 527,499
Cost of revenue	464,588	385,479	328,612
Gross profit	316,260	246,081	198,887
Operating expenses	270,222	214,672	184,568
Operating income	46,038	$ 31,409	$ 14,319
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue	(866)
Cost of revenue	(1,837)
Gross profit	971
Operating expenses	663
Operating income	308
Calculated under Revenue Guidance in Effect before Topic 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue	781,714
Cost of revenue	466,425
Gross profit	315,289
Operating expenses	269,559
Operating income	$ 45,730