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Fair Value Measurement - Marketable Securities Measured at Fair Value on Recurring Basis (Details) - Recurring - USD ($) $ in Thousands | Jun. 30, 2019 | Dec. 31, 2018
--- | --- | ---
Marketable securities |  |
Marketable securities | $ 103,336 | $ 119,961
Money market funds |  |
Marketable securities |  |
Marketable securities | 6,294 | 15,223
U.S. treasury securities |  |
Marketable securities |  |
Marketable securities | 77,591 | 66,489
Corporate bonds |  |
Marketable securities |  |
Marketable securities | 19,451 | 19,720
Commercial paper |  |
Marketable securities |  |
Marketable securities |  | 18,529
Level 1 |  |
Marketable securities |  |
Marketable securities | 83,885 | 81,712
Level 1 | Money market funds |  |
Marketable securities |  |
Marketable securities | 6,294 | 15,223
Level 1 | U.S. treasury securities |  |
Marketable securities |  |
Marketable securities | 77,591 | 66,489
Level 1 | Corporate bonds |  |
Marketable securities |  |
Marketable securities | 0 | 0
Level 1 | Commercial paper |  |
Marketable securities |  |
Marketable securities |  | 0
Level 2 |  |
Marketable securities |  |
Marketable securities | 19,451 | 38,249
Level 2 | Money market funds |  |
Marketable securities |  |
Marketable securities | 0 | 0
Level 2 | U.S. treasury securities |  |
Marketable securities |  |
Marketable securities | 0 | 0
Level 2 | Corporate bonds |  |
Marketable securities |  |
Marketable securities | 19,451 | 19,720
Level 2 | Commercial paper |  |
Marketable securities |  |
Marketable securities |  | 18,529
Level 3 |  |
Marketable securities |  |
Marketable securities | 0 | 0
Level 3 | Money market funds |  |
Marketable securities |  |
Marketable securities | 0 | 0
Level 3 | U.S. treasury securities |  |
Marketable securities |  |
Marketable securities | 0 | 0
Level 3 | Corporate bonds |  |
Marketable securities |  |
Marketable securities | $ 0 | 0
Level 3 | Commercial paper |  |
Marketable securities |  |
Marketable securities |  | $ 0

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- Definition Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -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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