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Summary of Significant Accounting Policies - Fair Value (Details) - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Fair Value of Financial Instruments |  |
Money Market Funds | $ 26,246 | $ 16,528
Securities | 504,223 |
Money Market Funds |  |
Fair Value of Financial Instruments |  |
Money Market Funds | 18,270 | 5,175
Corporate Securities |  |
Fair Value of Financial Instruments |  |
Securities | 104,967 | 123,270
Government Securities |  |
Fair Value of Financial Instruments |  |
Securities | 399,256 | 223,530
Fair Value, Measurements, Recurring |  |
Fair Value of Financial Instruments |  |
Total Fair Value | 522,493 | 351,975
Fair Value, Measurements, Recurring | Money Market Funds |  |
Fair Value of Financial Instruments |  |
Money Market Funds | 18,270 | 5,175
Fair Value, Measurements, Recurring | Corporate Securities |  |
Fair Value of Financial Instruments |  |
Securities | 104,967 | 123,270
Fair Value, Measurements, Recurring | Government Securities |  |
Fair Value of Financial Instruments |  |
Securities | 399,256 | 223,530
Level 1 | Fair Value, Measurements, Recurring |  |
Fair Value of Financial Instruments |  |
Total Fair Value | 18,270 | 5,175
Level 1 | Fair Value, Measurements, Recurring | Money Market Funds |  |
Fair Value of Financial Instruments |  |
Money Market Funds | 18,270 | 5,175
Level 2 | Fair Value, Measurements, Recurring |  |
Fair Value of Financial Instruments |  |
Total Fair Value | 504,223 | 346,800
Level 2 | Fair Value, Measurements, Recurring | Corporate Securities |  |
Fair Value of Financial Instruments |  |
Securities | 104,967 | 123,270
Level 2 | Fair Value, Measurements, Recurring | Government Securities |  |
Fair Value of Financial Instruments |  |
Securities | $ 399,256 | $ 223,530

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