Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Thousands		3 Months Ended			9 Months Ended		12 Months Ended
		Dec. 31, 2019	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2020
Contingencies And Commitments [Line Items]
Research and development expense	[1]		$ 57,246	$ 39,820	$ 130,601	$ 84,972
Research Agreements [Member]
Contingencies And Commitments [Line Items]
Research and development expense			200	$ 300	2,400	$ 1,400
Research Agreements [Member] | Scenario Forecast [Member]
Contingencies And Commitments [Line Items]
Research and development expense		$ 1,400					$ 2,300
Manufacturing Agreements [Member] | Prepaid Expenses [Member]
Contingencies And Commitments [Line Items]
Upfront payment recorded as prepaid expense			2,200		2,200
Letter of Credit [Member]
Contingencies And Commitments [Line Items]
Letters of credit secured by cash held in restricted depository account			$ 3,900		$ 3,900

[1] Including the following research and development expense with a related party, see Notes 6 & 11