Goodwill and Acquired Intangible Assets - Schedule of Changes in Goodwill by Reportable Segment (Details) - USD ($) $ in Thousands			3 Months Ended		12 Months Ended
	Nov. 01, 2019	Nov. 19, 2018	Sep. 30, 2020	Jun. 29, 2019	Sep. 30, 2020	Sep. 30, 2019
Goodwill [Line Items]
Balance, beginning of period					$ 1,238,179	$ 1,182,457
Foreign currency translation adjustments					10,435	(10,596)
Balance, end of period			$ 1,625,786		1,625,786	1,238,179
Frustum
Goodwill [Line Items]
Goodwill, acquired		$ 53,700				53,673
Other Acquisitions
Goodwill [Line Items]
Goodwill, acquired			12,300	$ 12,600	12,262	12,645
Onshape
Goodwill [Line Items]
Goodwill, acquired	$ 364,900				364,910
Software Products
Goodwill [Line Items]
Balance, beginning of period					1,196,064	1,152,720
Foreign currency translation adjustments					10,080	(10,329)
Balance, end of period			1,583,316		1,583,316	1,196,064
Software Products | Frustum
Goodwill [Line Items]
Goodwill, acquired						53,673
Software Products | Other Acquisitions
Goodwill [Line Items]
Goodwill, acquired					12,262	0
Software Products | Onshape
Goodwill [Line Items]
Goodwill, acquired					364,910
Professional Services
Goodwill [Line Items]
Balance, beginning of period					42,115	29,737
Foreign currency translation adjustments					355	(267)
Balance, end of period			$ 42,470		42,470	42,115
Professional Services | Frustum
Goodwill [Line Items]
Goodwill, acquired						0
Professional Services | Other Acquisitions
Goodwill [Line Items]
Goodwill, acquired					0	$ 12,645
Professional Services | Onshape
Goodwill [Line Items]
Goodwill, acquired					$ 0