Fair Value Measurements - Schedule of Financial Assets Measured at Fair Value on a Recurring Basis (Details) - Fair Value, Recurring - USD ($) $ in Thousands	Dec. 31, 2021	Sep. 30, 2021
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Cash equivalents, fair value	$ 20,044	$ 21,547
Investments, fair value	443,583	483,955
Certificates of Deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Debt Securities, Available-for-sale, Current	992	255
Corporate Bonds and Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Debt Securities, Available-for-sale, Current	177,093	186,107
Debt Securities, Available-for-sale, Noncurrent	48,638	53,107
Municipal Bonds and Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Debt Securities, Available-for-sale, Current	15,359	13,566
Debt Securities, Available-for-sale, Noncurrent	2,239	11,111
U.S. Government Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Debt Securities, Available-for-sale, Current	135,934	102,615
Debt Securities, Available-for-sale, Noncurrent	24,341	59,608
U.S. Government Agency Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Debt Securities, Available-for-sale, Current	17,170	27,087
Debt Securities, Available-for-sale, Noncurrent	1,773	8,952
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Cash equivalents, fair value	19,265	17,150
Investments, fair value	19,265	17,150
Level 1 | Certificates of Deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Debt Securities, Available-for-sale, Current	0	0
Level 1 | Corporate Bonds and Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Debt Securities, Available-for-sale, Current	0	0
Debt Securities, Available-for-sale, Noncurrent	0	0
Level 1 | Municipal Bonds and Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Debt Securities, Available-for-sale, Current	0	0
Debt Securities, Available-for-sale, Noncurrent	0	0
Level 1 | U.S. Government Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Debt Securities, Available-for-sale, Current	0	0
Debt Securities, Available-for-sale, Noncurrent	0	0
Level 1 | U.S. Government Agency Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Debt Securities, Available-for-sale, Current	0	0
Debt Securities, Available-for-sale, Noncurrent	0	0
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Cash equivalents, fair value	779	4,397
Investments, fair value	424,318	466,805
Level 2 | Certificates of Deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Debt Securities, Available-for-sale, Current	992	255
Level 2 | Corporate Bonds and Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Debt Securities, Available-for-sale, Current	177,093	186,107
Debt Securities, Available-for-sale, Noncurrent	48,638	53,107
Level 2 | Municipal Bonds and Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Debt Securities, Available-for-sale, Current	15,359	13,566
Debt Securities, Available-for-sale, Noncurrent	2,239	11,111
Level 2 | U.S. Government Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Debt Securities, Available-for-sale, Current	135,934	102,615
Debt Securities, Available-for-sale, Noncurrent	24,341	59,608
Level 2 | U.S. Government Agency Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Debt Securities, Available-for-sale, Current	17,170	27,087
Debt Securities, Available-for-sale, Noncurrent	1,773	8,952
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Cash equivalents, fair value	0	0
Investments, fair value	0	0
Level 3 | Certificates of Deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Debt Securities, Available-for-sale, Current	0	0
Level 3 | Corporate Bonds and Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Debt Securities, Available-for-sale, Current	0	0
Debt Securities, Available-for-sale, Noncurrent	0	0
Level 3 | Municipal Bonds and Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Debt Securities, Available-for-sale, Current	0	0
Debt Securities, Available-for-sale, Noncurrent	0	0
Level 3 | U.S. Government Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Debt Securities, Available-for-sale, Current	0	0
Debt Securities, Available-for-sale, Noncurrent	0	0
Level 3 | U.S. Government Agency Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Debt Securities, Available-for-sale, Current	0	0
Debt Securities, Available-for-sale, Noncurrent	$ 0	$ 0