Net Loss Per Share Attributable to Common Stockholders - Computation of the Company's Basic and Diluted Net Loss Per Share Attributable to Common Stockholders (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Numerator:
Net loss attributable to common stockholders	$ (139,931)	$ (107,669)	$ (71,236)
Denominator:
Weighted-average shares used in computing net loss per share, basic and diluted	39,190	27,462	2,793
Net loss per share attributable to common stockholders, basic and diluted	$ (3.57)	$ (3.92)	$ (25.51)