Revenue from Contracts with Customers - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Disaggregation Of Revenue [Line Items]
Revenue	$ 1,458,415	$ 1,255,631	$ 1,241,824
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Disaggregation Of Revenue [Line Items]
Revenue		1,311,494	1,241,824
Total recurring revenue
Disaggregation Of Revenue [Line Items]
Revenue	1,281,949	1,017,398
Total recurring revenue | Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Disaggregation Of Revenue [Line Items]
Revenue		1,078,627	978,853
Perpetual license
Disaggregation Of Revenue [Line Items]
Revenue	32,668	70,702
Perpetual license | Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Disaggregation Of Revenue [Line Items]
Revenue		72,191	109,634
Professional services
Disaggregation Of Revenue [Line Items]
Revenue	$ 143,798	167,531	153,337
Professional services | Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Disaggregation Of Revenue [Line Items]
Revenue		$ 160,676	$ 153,337