Fair Value Measurements (Schedule of Financial Assets Measured at Fair Value on a Recurring Basis) (Details) - Fair Value, Recurring - USD ($) $ in Thousands	Dec. 31, 2019	Sep. 30, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Cash equivalents, fair value	$ 291,565	$ 293,642
Investments, fair value	1,030,370	1,025,107
Certificates of Deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Debt Securities, Available-for-sale, Current	991	249
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	264,425	259,547
Debt Securities, Available-for-sale, Noncurrent	274,477	298,916
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	9,190	12,129
Debt Securities, Available-for-sale, Noncurrent		2,524
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	131,808	78,992
Debt Securities, Available-for-sale, Noncurrent	7,559	5,515
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	20,347	22,146
Debt Securities, Available-for-sale, Noncurrent	30,008	51,447
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Cash equivalents, fair value	249,178	140,238
Debt Securities, Available-for-sale, Noncurrent	0	0
Investments, fair value	249,178	140,238
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
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
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Cash equivalents, fair value	42,387	153,404
Investments, fair value	781,192	884,869
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	991	249
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	264,425	259,547
Debt Securities, Available-for-sale, Noncurrent	274,477	298,916
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	9,190	12,129
Debt Securities, Available-for-sale, Noncurrent		2,524
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	131,808	78,992
Debt Securities, Available-for-sale, Noncurrent	7,559	5,515
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	20,347	22,146
Debt Securities, Available-for-sale, Noncurrent	30,008	51,447
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Cash equivalents, fair value	0	0
Debt Securities, Available-for-sale, Noncurrent	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
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