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Fair Value Measurements - Fair Value Hierarchy (Details) - USD ($) $ in Thousands | Jun. 30, 2017 | Dec. 31, 2016
--- | --- | ---
Assets |  |
Total assets | $ 90,863 | $ 89,348
Fair Value, Inputs, Level 1 |  |
Assets |  |
Total assets | 443 | 80
Fair Value, Inputs, Level 2 |  |
Assets |  |
Total assets | 90,420 | 89,268
Money market funds |  |
Assets |  |
Total assets | 443 | 80
Money market funds | Fair Value, Inputs, Level 1 |  |
Assets |  |
Total assets | 443 | 80
U.S. Government agency bonds |  |
Assets |  |
Total assets | 3,686 | 9,515
U.S. Government agency bonds | Fair Value, Inputs, Level 2 |  |
Assets |  |
Total assets | 3,686 | 9,515
U.S. Government bonds |  |
Assets |  |
Total assets | 1,787 |
U.S. Government bonds | Fair Value, Inputs, Level 2 |  |
Assets |  |
Total assets | 1,787 |
Bank certificates of deposit |  |
Assets |  |
Total assets | 15,458 | 11,113
Bank certificates of deposit | Fair Value, Inputs, Level 2 |  |
Assets |  |
Total assets | 15,458 | 11,113
Commercial paper. |  |
Assets |  |
Total assets | 8,328 | 17,010
Commercial paper. | Fair Value, Inputs, Level 2 |  |
Assets |  |
Total assets | 8,328 | 17,010
Corporate notes |  |
Assets |  |
Total assets | 53,416 | 45,124
Corporate notes | Fair Value, Inputs, Level 2 |  |
Assets |  |
Total assets | 53,416 | 45,124
Asset-backed securities |  |
Assets |  |
Total assets | 7,745 | 6,506
Asset-backed securities | Fair Value, Inputs, Level 2 |  |
Assets |  |
Total assets | $ 7,745 | $ 6,506

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