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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

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