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Investments - Available-for-sale Securities (Details) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Total available-for-sale securities |  |
Estimated Fair Value | $ 44,781 | $ 43,787
Gross Unrealized Losses | (27) | (20)
Short-term investments that had been in a continuous loss position for more than 12 months | 0 | 0
U.S. treasuries |  |
Total available-for-sale securities |  |
Estimated Fair Value | 33,895 | 20,002
Gross Unrealized Losses | (19) | (5)
Government agency securities |  |
Total available-for-sale securities |  |
Estimated Fair Value |  | 10,012
Gross Unrealized Losses |  | (8)
Corporate debt |  |
Total available-for-sale securities |  |
Estimated Fair Value | 10,886 | 13,773
Gross Unrealized Losses | $ (8) | $ (7)

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