Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Series A Redeemable Convertible Preferred Stock [Member]	Series B Redeemable Convertible Preferred Stock [Member]	Series C Redeemable Convertible Preferred Stock [Member]	Series D Redeemable Convertible Preferred Stock [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income [Member]	Accumulated deficit [Member]
Beginning Balance at Sep. 30, 2017		$ 9,141	$ 25,900	$ 36,726	$ 127,866
Beginning Balance , shares at Sep. 30, 2017		2,818	3,316	2,491	6,034
Issuance of Series D redeemable convertible preferred stock, net of financing costs					$ 90,850
Issuance of Series D redeemable convertible preferred stock, net of financing costs, shares					4,292
Ending Balance at Sep. 30, 2018		$ 9,141	$ 25,900	$ 36,726	$ 218,716
Ending Balance , shares at Sep. 30, 2018		2,818	3,316	2,491	10,326
Beginning Balance at Sep. 30, 2017	$ (133,358)						$ 6,228	$ 33	$ (139,619)
Beginning Balance , shares at Sep. 30, 2017						3,179
Exercise of stock options	215						215
Exercise of stock options , shares						63
Repurchase of early exercised stock options , shares						(10)
Stock-based compensation	2,961						2,961
Forfeiture of restricted common stock , shares						(21)
Treasury stock purchase, Shares						(5)
Treasury stock purchase	(58)						(58)
Other comprehensive income	54							54
Net loss	(71,236)								(71,236)
Ending Balance at Sep. 30, 2018	$ (201,422)						9,346	87	(210,855)
Ending Balance , shares at Sep. 30, 2018						3,206
Issuance of Series D redeemable convertible preferred stock, net of financing costs, shares					0
Ending Balance at Sep. 30, 2019		$ 0	$ 0	$ 0	$ 0
Ending Balance , shares at Sep. 30, 2019	0
Issuance of common stock in public offering	$ 153,852				0		153,852
Issuance of common stock in public offering, shares						10,063
Vesting of restricted stock units	0						0
Vesting of restricted stock units, shares						8
Issuance of shares under the employee stock purchase plan	2,700						2,700
Issuance of shares under the employee stock purchase plan, shares						219
Exercise of stock options	2,264						2,264
Exercise of stock options , shares						331
Conversion of redeemable convertible preferred stock warrant liability to equity	631						631
Conversion of redeemable convertible preferred stock to common stock	290,462	$ (9,141)	$ (25,900)	$ (36,726)	$ (218,716)		290,462
Conversion of redeemable convertible preferred stock to common stock , shares		(2,818)	(3,316)	(2,491)	(10,326)	18,951
Stock-based compensation	11,170						11,170
Net exercise of stock warrants , shares						95
Other comprehensive income	94							94
Net loss	(107,669)								(107,669)
Ending Balance at Sep. 30, 2019	152,082						470,425	181	(318,524)
Ending Balance , shares at Sep. 30, 2019						32,873
Issuance of common stock in public offering	295,563						295,563
Issuance of common stock in public offering, shares						11,064
Vesting of restricted stock units, shares						178
Issuance of shares under the employee stock purchase plan	3,428						3,428
Issuance of shares under the employee stock purchase plan, shares						126
Exercise of stock options	10,539						10,539
Exercise of stock options , shares						915
Stock-based compensation	17,096						17,096
Other comprehensive income	(94)							(94)
Repurchases of common stock for income tax withholding	(2,421)						(2,421)
Repurchases of common stock for income tax withholding , shares						(71)
Net loss	(139,931)								(139,931)
Ending Balance at Sep. 30, 2020	$ 336,262						$ 794,630	$ 87	$ (458,455)
Ending Balance , shares at Sep. 30, 2020						45,083