Fair value measurement - Cash And Cash Equivalents And Available For Sale Securities At Fair Value (Detail) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Cash And Cash Equivalents At Amortized Cost	$ 93,667	$ 46,735
Available-for-sale Securities, Amortized Cost Basis	196,320	91,323
Cash And Cash Equivalents And Short Term Investments Amortized Cost	289,987	138,058
Available-for-sale Securities, Gross Unrealized Gain	15	49
Available for sale Securities Gross Unrealized Gain Loss	15	49
Cash and Cash Equivalents, at Carrying Value	93,667	46,735	$ 80,757
Short-term Investments	196,335	91,372
Cash, Cash Equivalents, and Short-term Investments	$ 290,002	$ 138,107