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Fair Value Measurements - Schedule of Financial Assets Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands | Mar. 31, 2022 | Sep. 30, 2021
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items] |  |
Cash equivalents, fair value | $ 8,220 | $ 21,547
Cost or amortized cost | 344,394 | 483,931
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 2 | 145
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | (2,174) | (121)
Fair value | 342,222 | 483,955
Debt Securities, Available-for-sale, Current | 300,591 | 329,630
Debt Securities, Available-for-sale, Noncurrent | 33,411 | 132,778
Equity Securities, FV-NI | 1,500 |
Equity Securities, FV-NI, Current | 0 |
Equity Securities, FV-NI, Noncurrent | 1,500 |
Available-for-sale Securities | 343,722 |
Available-for-sale Securities, Current | 300,591 | 329,630
Long-term investments | 34,911 | 132,778
Certificates of Deposit |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items] |  |
Cost or amortized cost | 991 | 255
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 0 | 0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 0 | 0
Corporate Bonds and Notes |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items] |  |
Cost or amortized cost | 147,313 | 243,568
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 2 | 129
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | (1,063) | (86)
Municipal Bonds and Notes |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items] |  |
Cost or amortized cost | 18,779 | 24,684
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 0 | 2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | (143) | (9)
U.S. Government Securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items] |  |
Cost or amortized cost | 158,023 | 162,221
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 0 | 14
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | (891) | (12)
U.S. Government Agency Securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items] |  |
Cost or amortized cost | 11,068 | 36,053
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 0 | 0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | (77) | (14)
Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items] |  |
Cash equivalents, fair value |  | 17,150
Fair Value, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items] |  |
Equity Securities, FV-NI | 1,500 |
Equity Securities, FV-NI, Current | 0 |
Equity Securities, FV-NI, Noncurrent | 1,500 |
Fair Value, Recurring | Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items] |  |
Cash equivalents, fair value | 8,220 | 17,150
Fair Value, Recurring | Level 2 | Certificates of Deposit |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items] |  |
Fair value | 991 | 255
Debt Securities, Available-for-sale, Current | 991 | 255
Debt Securities, Available-for-sale, Noncurrent | 0 | 0
Fair Value, Recurring | Level 2 | Corporate Bonds and Notes |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items] |  |
Cash equivalents, fair value | 0 | 4,397
Fair value | 146,252 | 243,611
Debt Securities, Available-for-sale, Current | 119,576 | 186,107
Debt Securities, Available-for-sale, Noncurrent | 26,676 | 53,107
Fair Value, Recurring | Level 2 | Municipal Bonds and Notes |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items] |  |
Fair value | 18,636 | 24,677
Debt Securities, Available-for-sale, Current | 16,458 | 13,566
Debt Securities, Available-for-sale, Noncurrent | 2,178 | 11,111
Fair Value, Recurring | Level 2 | U.S. Government Securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items] |  |
Fair value | 157,132 | 162,223
Debt Securities, Available-for-sale, Current | 154,313 | 102,615
Debt Securities, Available-for-sale, Noncurrent | 2,819 | 59,608
Fair Value, Recurring | Level 2 | U.S. Government Agency Securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items] |  |
Fair value | 10,991 | 36,039
Debt Securities, Available-for-sale, Current | 9,253 | 27,087
Debt Securities, Available-for-sale, Noncurrent | $ 1,738 | $ 8,952

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