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Financial Instruments (Details) $ in Thousands | 9 Months Ended |
--- | --- | ---
Sep. 30, 2021 USD ($) position | Sep. 30, 2020 USD ($) | Dec. 31, 2020 USD ($)
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract] |  |  |
Estimated Fair Value | $ 928,596 |  | $ 688,606
Proceeds from investments sold | 77,600 | $ 11,500 |
Other than temporary impairment | $ 0 |  |
Number of investments, unrealized loss position | position | 43 |  |
Amortized Cost |  |  |
Less than or equal to one year | $ 403,211 |  |
Greater than one year but less than five years | 523,885 |  |
Total | 927,096 |  |
Fair Value |  |  |
Less than or equal to one year | 403,784 |  |
Greater than or equal to one year but less than five years | 524,812 |  |
Total | 928,596 |  |
Available-for-sale securities |  |  |
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract] |  |  |
Amortized Cost | 982,799 |  | 713,337
Gross Unrealized Gain | 2,199 |  | 4,310
Gross Unrealized Loss | (699) |  | (51)
Estimated Fair Value | 984,299 |  | 717,596
Unrealized loss position | 522,000 |  |
Money market deposits |  |  |
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract] |  |  |
Amortized Cost | 55,703 |  | 28,990
Estimated Fair Value | 55,703 |  | 28,990
U.S. Treasury securities |  |  |
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract] |  |  |
Amortized Cost | 786,404 |  | 594,252
Gross Unrealized Gain | 1,821 |  | 3,512
Gross Unrealized Loss | (350) |  | (20)
Estimated Fair Value | 787,875 |  | 597,744
Corporate bonds and notes |  |  |
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract] |  |  |
Amortized Cost | 52,868 |  | 12,331
Gross Unrealized Gain |  |  | 2
Gross Unrealized Loss | (147) |  | (5)
Estimated Fair Value | 52,721 |  | 12,328
Municipal securities |  |  |
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract] |  |  |
Amortized Cost | 87,824 |  | 77,764
Gross Unrealized Gain | 378 |  | 796
Gross Unrealized Loss | (202) |  | (26)
Estimated Fair Value | 88,000 |  | 78,534
Cash equivalents | Available-for-sale securities |  |  |
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract] |  |  |
Estimated Fair Value | 55,703 |  | 28,990
Short-term investments | Available-for-sale securities |  |  |
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract] |  |  |
Estimated Fair Value | $ 928,596 |  | $ 688,606

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