Fair Value of Financial Instruments (Details) - Fair Value Measurements, Recurring Basis - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Assets:
Cash and cash equivalents	$ 96,800	$ 62,500
Total assets measured at fair value	156,484	155,910
Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Total assets measured at fair value	96,851	61,512
Significant Other Observable Inputs (Level 2)
Assets:
Total assets measured at fair value	59,633	94,398
Significant Unobservable Inputs (Level 3)
Assets:
Total assets measured at fair value	0	0
Money market funds
Assets:
Cash and cash equivalents	96,851	61,512
Money market funds \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Cash and cash equivalents	96,851	61,512
Money market funds \| Significant Other Observable Inputs (Level 2)
Assets:
Cash and cash equivalents	0	0
Money market funds \| Significant Unobservable Inputs (Level 3)
Assets:
Cash and cash equivalents	0	0
U.S. Treasury securities
Assets:
Available-for-sale securities	3,997	3,990
U.S. Treasury securities \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Available-for-sale securities	0	0
U.S. Treasury securities \| Significant Other Observable Inputs (Level 2)
Assets:
Available-for-sale securities	3,997	3,990
U.S. Treasury securities \| Significant Unobservable Inputs (Level 3)
Assets:
Available-for-sale securities	0	0
Government-sponsored enterprises
Assets:
Available-for-sale securities		11,990
Government-sponsored enterprises \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Available-for-sale securities		0
Government-sponsored enterprises \| Significant Other Observable Inputs (Level 2)
Assets:
Available-for-sale securities		11,990
Government-sponsored enterprises \| Significant Unobservable Inputs (Level 3)
Assets:
Available-for-sale securities		0
Corporate debt securities
Assets:
Available-for-sale securities	55,636	78,418
Corporate debt securities \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Available-for-sale securities	0	0
Corporate debt securities \| Significant Other Observable Inputs (Level 2)
Assets:
Available-for-sale securities	55,636	78,418
Corporate debt securities \| Significant Unobservable Inputs (Level 3)
Assets:
Available-for-sale securities	$ 0	$ 0

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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Amount of investment in debt and equity securities categorized neither as trading securities nor held-to-maturity securities and intended be sold or mature one year or operating cycle, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26626-111562

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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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