Fair Value Measurement - Summary of Contractual Maturities of Available for Sale Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Sep. 30, 2020
Schedule Of Available For Sale Securities [Line Items]
Available for sale securities, contractual maturities less than 1 year	$ 138,534	$ 86,186
Available for sale securities, contractual maturities between 1 to 5 years	81,754	87,596
Available for sale securities, contractual maturities between 5 to 10 years	56,462	151,042
Available for sale securities, contractual maturities after 10 years	36,793	34,793
Available for sale securities, contractual maturities, Total	313,543	359,617
Corporate Bonds [Member]
Schedule Of Available For Sale Securities [Line Items]
Available for sale securities, contractual maturities less than 1 year	49,308	44,701
Available for sale securities, contractual maturities between 1 to 5 years	61,315	68,171
Available for sale securities, contractual maturities between 5 to 10 years	34,172	95,969
Available for sale securities, contractual maturities, Total	144,795	208,841
Money Market Funds [Member]
Schedule Of Available For Sale Securities [Line Items]
Available for sale securities, contractual maturities less than 1 year	82,066	39,519
Available for sale securities, contractual maturities, Total	82,066	39,519
U.S. Government Agency Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Available for sale securities, contractual maturities less than 1 year	1,013	1,007
Available for sale securities, contractual maturities between 1 to 5 years	18,668	18,497
Available for sale securities, contractual maturities between 5 to 10 years	20,670	25,181
Available for sale securities, contractual maturities, Total	40,351	44,685
Asset Backed Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Available for sale securities, contractual maturities less than 1 year	2
Available for sale securities, contractual maturities between 1 to 5 years	267	246
Available for sale securities, contractual maturities between 5 to 10 years		2,201
Available for sale securities, contractual maturities after 10 years	36,793	34,793
Available for sale securities, contractual maturities, Total	37,062	37,240
U.S. Treasuries [Member]
Schedule Of Available For Sale Securities [Line Items]
Available for sale securities, contractual maturities less than 1 year	1,367	959
Available for sale securities, contractual maturities between 1 to 5 years	1,504	682
Available for sale securities, contractual maturities between 5 to 10 years	1,620	27,691
Available for sale securities, contractual maturities, Total	4,491	$ 29,332
Supranational Bonds [Member]
Schedule Of Available For Sale Securities [Line Items]
Available for sale securities, contractual maturities less than 1 year	4,778
Available for sale securities, contractual maturities, Total	$ 4,778

Fair value of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in sixth through tenth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

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