Fair Value Measurement - (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Sep. 30, 2020
Assets
Assets, Totals	$ 555,658	$ 290,002
Commercial Paper [Member]
Assets
Assets, Totals	74,958	94,840
US Government Treasury Bills [Member]
Assets
Assets, Totals	81,371	101,495
Cash [Member]
Assets
Assets, Totals	33,515	20,254
Money Market Funds [Member]
Assets
Assets, Totals	365,814	73,413
Fair Value, Inputs, Level 1 [Member]
Assets
Assets, Totals	480,700	195,162
Fair Value, Inputs, Level 1 [Member] | US Government Treasury Bills [Member]
Assets
Assets, Totals	81,371	101,495
Fair Value, Inputs, Level 1 [Member] | Cash [Member]
Assets
Assets, Totals	33,515	20,254
Fair Value, Inputs, Level 1 [Member] | Money Market Funds [Member]
Assets
Assets, Totals	365,814	73,413
Fair Value, Inputs, Level 2 [Member]
Assets
Assets, Totals	74,958	94,840
Fair Value, Inputs, Level 2 [Member] | Commercial Paper [Member]
Assets
Assets, Totals	$ 74,958	$ 94,840