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Marketable Securities - Summary of Available for Sale Securities (Details) $ in Thousands | Sep. 30, 2021 USD ($) security | Dec. 31, 2020 USD ($) security
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | $ 964,265 | $ 995,147
Unrealized Holding Gains | 37 | 130
Unrealized Holding Losses | (208) | (25)
Aggregate Fair Value | 964,094 | 995,252
Short-term marketable securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 644,627 | 962,439
Unrealized Holding Gains | 18 | 128
Unrealized Holding Losses | (23) | (14)
Aggregate Fair Value | 644,622 | 962,553
Short-term marketable securities | U.S. government treasuries |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 472,214 | 878,906
Unrealized Holding Gains | 13 | 44
Unrealized Holding Losses | (13) | (12)
Aggregate Fair Value | $ 472,214 | $ 878,938
Number of securities held in unrealized holding loss position | security | 7 | 19
Aggregate fair value, unrealized holding loss position | $ 271,800 | $ 369,900
Short-term marketable securities | U.S. government agency securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 15,003 | 25,214
Unrealized Holding Gains | 0 | 5
Unrealized Holding Losses | 0 | (2)
Aggregate Fair Value | 15,003 | $ 25,217
Number of securities held in unrealized holding loss position | security |  | 2
Aggregate fair value, unrealized holding loss position |  | $ 10,100
Short-term marketable securities | Corporate debt securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 61,187 | 27,101
Unrealized Holding Gains | 5 | 79
Unrealized Holding Losses | (10) | 0
Aggregate Fair Value | $ 61,182 | 27,180
Number of securities held in unrealized holding loss position | security | 8 |
Aggregate fair value, unrealized holding loss position | $ 27,800 |
Short-term marketable securities | Commercial paper |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 96,223 | 31,218
Unrealized Holding Gains | 0 | 0
Unrealized Holding Losses | 0 | 0
Aggregate Fair Value | 96,223 | 31,218
Long-term marketable securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 319,638 | 32,708
Unrealized Holding Gains | 19 | 2
Unrealized Holding Losses | (185) | (11)
Aggregate Fair Value | 319,472 | 32,699
Long-term marketable securities | U.S. government treasuries |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 299,225 | 2,561
Unrealized Holding Gains | 17 | 0
Unrealized Holding Losses | (173) | 0
Aggregate Fair Value | $ 299,069 | 2,561
Number of securities held in unrealized holding loss position | security | 11 |
Aggregate fair value, unrealized holding loss position | $ 251,500 |
Long-term marketable securities | Corporate debt securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 20,413 | 30,147
Unrealized Holding Gains | 2 | 2
Unrealized Holding Losses | (12) | (11)
Aggregate Fair Value | $ 20,403 | $ 30,138
Number of securities held in unrealized holding loss position | security | 5 | 1
Aggregate fair value, unrealized holding loss position | $ 16,800 | $ 20,100

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