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Assets - Available-For-Sale Securities (Details) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Jun. 30, 2019 | Jun. 30, 2018 | Jun. 30, 2019 | Jun. 30, 2018 | Dec. 31, 2018
Total available-for-sale securities |  |  |  |  |
Amortized Cost | $ 84,466 |  | $ 84,466 |  | $ 48,205
Gross Unrealized Gains | 29 |  | 29 |  |
Gross Unrealized Losses |  |  |  |  | (16)
Estimated Fair Value | 84,495 |  | 84,495 |  | 48,189
Available-for-sale securities in a loss position |  |  |  |  |
Estimated Fair Value |  |  |  |  | 45,700
Gross Unrealized Losses |  |  |  |  | (16)
Investments that had been in a continuous loss position for more than 12 months | 0 |  | 0 |  | 0
Realized gains or losses |  |  |  |  |
Realized gains | 0 | $ 0 | 0 | $ 0 |
Realized losses | 0 | $ 0 |  |  |
U.S. treasuries |  |  |  |  |
Total available-for-sale securities |  |  |  |  |
Amortized Cost | 56,877 |  | 56,877 |  | 37,191
Gross Unrealized Gains | 28 |  | 28 |  |
Gross Unrealized Losses |  |  |  |  | (7)
Estimated Fair Value | 56,905 |  | 56,905 |  | 37,184
Available-for-sale securities in a loss position |  |  |  |  |
Estimated Fair Value |  |  |  |  | 37,184
Gross Unrealized Losses |  |  |  |  | (7)
Government agency securities |  |  |  |  |
Total available-for-sale securities |  |  |  |  |
Amortized Cost | 4,995 |  | 4,995 |  |
Estimated Fair Value | 4,995 |  | 4,995 |  |
Corporate debt |  |  |  |  |
Total available-for-sale securities |  |  |  |  |
Amortized Cost | 1,001 |  | 1,001 |  | 8,525
Gross Unrealized Losses |  |  |  |  | (9)
Estimated Fair Value | 1,001 |  | 1,001 |  | 8,516
Available-for-sale securities in a loss position |  |  |  |  |
Estimated Fair Value |  |  |  |  | 8,516
Gross Unrealized Losses |  |  |  |  | (9)
Commercial paper |  |  |  |  |
Total available-for-sale securities |  |  |  |  |
Amortized Cost | 21,593 |  | 21,593 |  | 2,489
Gross Unrealized Gains | 1 |  | 1 |  |
Estimated Fair Value | $ 21,594 |  | $ 21,594 |  | $ 2,489

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