Segment and Geographic Information - Summary of Revenue and Profit Attributable to Our Operating Segments (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Revenue Reconciling Item [Line Items]
Revenue	$ 1,458,415	$ 1,255,631	$ 1,241,824
Operating costs	334,271	325,378	326,502
Gross margin	1,124,144	930,253	915,322
Sales and marketing	435,451	417,449	414,764
General and administrative	159,826	127,919	143,045
Restructuring and other charges, net	32,716	51,114	3,764
Total operating income	210,863	63,042	72,613
Interest expense	(76,428)	(43,047)	(41,673)
Other income (expense), net	271	305	(2,284)
Income before income taxes	134,706	20,300	28,656
Calculated under Revenue Guidance in Effect before Topic 606
Segment Reporting Revenue Reconciling Item [Line Items]
Revenue		1,311,494	1,241,824
Total operating income		101,620
Interest expense		(43,047)
Other income (expense), net		131
Income before income taxes		58,704
Operating Segments
Segment Reporting Revenue Reconciling Item [Line Items]
Revenue	1,458,415	1,255,631	1,241,824
Operating costs	522,481	511,310	524,953
Gross margin	935,934	744,321	716,871
Operating Segments | Software Products
Segment Reporting Revenue Reconciling Item [Line Items]
Revenue	1,314,617	1,088,100	1,088,487
Operating costs	393,803	377,464	387,989
Gross margin	920,814	710,636	700,498
Operating Segments | Professional Services
Segment Reporting Revenue Reconciling Item [Line Items]
Revenue	143,798	167,531	153,337
Operating costs	128,678	133,846	136,964
Gross margin	15,120	33,685	16,373
Operating Segments | Calculated under Revenue Guidance in Effect before Topic 606
Segment Reporting Revenue Reconciling Item [Line Items]
Revenue		1,311,494
Operating costs		504,086
Gross margin		807,408
Operating Segments | Calculated under Revenue Guidance in Effect before Topic 606 | Software Products
Segment Reporting Revenue Reconciling Item [Line Items]
Revenue		1,150,818
Operating costs		375,268
Gross margin		775,550
Operating Segments | Calculated under Revenue Guidance in Effect before Topic 606 | Professional Services
Segment Reporting Revenue Reconciling Item [Line Items]
Revenue		160,676
Operating costs		128,818
Gross margin		31,858
Unallocated
Segment Reporting Revenue Reconciling Item [Line Items]
Sales and marketing	398,100	385,423	389,871
General and administrative	114,386	104,393	108,159
Intangibles amortization	56,104	51,147	58,056
Restructuring and other charges, net	32,716	51,114	3,764
Stock-based compensation	115,149	86,400	82,939
Other unallocated operating expenses	$ 8,616	2,802	$ 1,469
Unallocated | Calculated under Revenue Guidance in Effect before Topic 606
Segment Reporting Revenue Reconciling Item [Line Items]
Sales and marketing		409,932
General and administrative		104,393
Intangibles amortization		51,147
Restructuring and other charges, net		51,114
Stock-based compensation		86,400
Other unallocated operating expenses		$ 2,802