Fair Value Measurements - Schedule of Financial Assets Measured at Fair Value on a Recurring Basis (Details) - Fair Value, Recurring - USD ($) $ in Thousands	Dec. 31, 2020	Sep. 30, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Cash equivalents, fair value	$ 494,901	$ 250,970
Investments, fair value	930,169	714,242
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	179,694	189,662
Debt Securities, Available-for-sale, Noncurrent	72,173	102,939
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	2,916	6,146
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	116,526	117,374
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	47,379	47,151
Debt Securities, Available-for-sale, Noncurrent	16,580
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Cash equivalents, fair value	286,495	43,553
Investments, fair value	286,495	43,553
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
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
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
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Cash equivalents, fair value	208,406	207,417
Investments, fair value	643,674	670,689
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	179,694	189,662
Debt Securities, Available-for-sale, Noncurrent	72,173	102,939
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	2,916	6,146
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	116,526	117,374
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	47,379	47,151
Debt Securities, Available-for-sale, Noncurrent	16,580
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 | 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
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
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