Relationship with Parent and Related Entities - Summary of Components Net Transfers to Parent (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Relationship With Parent And Related Entities Disclosure [Line Items]
Stock-based compensation	$ 47,285	$ 29,682	$ 22,043
Parent
Relationship With Parent And Related Entities Disclosure [Line Items]
Net transactions with Parent	(6,098)	(83,554)	(28,947)
Distribution to Parent	(152,978)
Net reclassification of net parent investment in Cerence	(938,051)
Stock-based compensation		29,682	22,043
Accrued bonus		9,478	(2,859)
Corporate depreciation and amortization		1,659	2,617
Fixed asset reclasses from the Parent		10,088	259
Voicebox Purchase Accounting Adjustment		3,591
Intangible asset reclasses from the Parent		1,665
Net transfer to Parent	$ (1,097,127)	$ (27,391)	$ (6,887)