Summary of Significant Accounting Policies - Summary of Fair Value Measurement Financial Asset and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Assets [Abstract]
Cash and cash equivalents	$ 146,200	$ 62,500
Securities		93,394
Common stock warrants	1,890
Total assets measured at fair value	160,849	155,910
Money market funds
Assets [Abstract]
Cash and cash equivalents	46,257	61,512
U.S Treasury securities
Assets [Abstract]
Marketable securities		3,989
Securities	12,488
Government-sponsored enterprises
Assets [Abstract]
Marketable securities		11,991
Corporate debt securities
Assets [Abstract]
Marketable securities		78,418
Securities	100,214
Quoted Prices in Active Markets for Identical Assets, Level 1
Assets [Abstract]
Common stock warrants	0
Total assets measured at fair value	46,257	61,512
Quoted Prices in Active Markets for Identical Assets, Level 1 \| Money market funds
Assets [Abstract]
Cash and cash equivalents	46,257	61,512
Quoted Prices in Active Markets for Identical Assets, Level 1 \| U.S Treasury securities
Assets [Abstract]
Marketable securities		0
Securities	0
Quoted Prices in Active Markets for Identical Assets, Level 1 \| Government-sponsored enterprises
Assets [Abstract]
Marketable securities		0
Quoted Prices in Active Markets for Identical Assets, Level 1 \| Corporate debt securities
Assets [Abstract]
Marketable securities		0
Securities	0
Significant Other Observable Inputs, Level 2
Assets [Abstract]
Common stock warrants	0
Total assets measured at fair value	112,702	94,398
Significant Other Observable Inputs, Level 2 \| Money market funds
Assets [Abstract]
Cash and cash equivalents	0	0
Significant Other Observable Inputs, Level 2 \| U.S Treasury securities
Assets [Abstract]
Marketable securities		3,989
Securities	12,488
Significant Other Observable Inputs, Level 2 \| Government-sponsored enterprises
Assets [Abstract]
Marketable securities		11,991
Significant Other Observable Inputs, Level 2 \| Corporate debt securities
Assets [Abstract]
Marketable securities		78,418
Securities	100,214
Significant Unobservable Inputs, Level 3
Assets [Abstract]
Common stock warrants	1,890
Total assets measured at fair value	1,890	0
Significant Unobservable Inputs, Level 3 \| Money market funds
Assets [Abstract]
Cash and cash equivalents	0	0
Significant Unobservable Inputs, Level 3 \| U.S Treasury securities
Assets [Abstract]
Marketable securities		0
Securities	0
Significant Unobservable Inputs, Level 3 \| Government-sponsored enterprises
Assets [Abstract]
Marketable securities		0
Significant Unobservable Inputs, Level 3 \| Corporate debt securities
Assets [Abstract]
Marketable securities		$ 0
Securities	$ 0

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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
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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
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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.

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