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Fair Value Measurements - Fair Value Hierarchy (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Assets |  |
Total assets | $ 112,823 | $ 99,876
Liabilities |  |
Total liabilities | 0 | 0
Fair Value, Inputs, Level 1 |  |
Assets |  |
Total assets | 1,156 | 2,370
Fair Value, Inputs, Level 2 |  |
Assets |  |
Total assets | 111,667 | 97,506
Money market funds |  |
Assets |  |
Total assets | 1,156 | 2,370
Money market funds | Fair Value, Inputs, Level 1 |  |
Assets |  |
Total assets | 1,156 | 2,370
U.S. Government agency bonds |  |
Assets |  |
Total assets | 1,982 | 2,681
U.S. Government agency bonds | Fair Value, Inputs, Level 2 |  |
Assets |  |
Total assets | 1,982 | 2,681
U.S. Government bonds |  |
Assets |  |
Total assets | 1,297 | 1,782
U.S. Government bonds | Fair Value, Inputs, Level 2 |  |
Assets |  |
Total assets | 1,297 | 1,782
Bank certificates of deposit |  |
Assets |  |
Total assets | 15,201 | 10,298
Bank certificates of deposit | Fair Value, Inputs, Level 2 |  |
Assets |  |
Total assets | 15,201 | 10,298
Commercial paper. |  |
Assets |  |
Cash equivalents |  | 3,000
Total assets | 9,593 | 14,593
Commercial paper. | Fair Value, Inputs, Level 2 |  |
Assets |  |
Total assets | 9,593 | 14,593
Corporate notes |  |
Assets |  |
Cash equivalents | 1,000 |
Total assets | 61,752 | 51,417
Corporate notes | Fair Value, Inputs, Level 2 |  |
Assets |  |
Total assets | 61,752 | 51,417
Asset-backed securities |  |
Assets |  |
Total assets | 21,842 | 16,735
Asset-backed securities | Fair Value, Inputs, Level 2 |  |
Assets |  |
Total assets | $ 21,842 | $ 16,735

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- Definition Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258 + Details
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- Definition Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. + References No definition available. + Details
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