Cash, Cash Equivalents and Short-term Investments (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Cash and cash equivalents
Amortized Cost	$ 67,868
Gross Unrealized Gains	0
Gross Unrealized Loss	0
Fair Value	67,868
Short-term Investments [Abstract]
Amortized Cost		$ 56,689
Gross Unrealized Gains		0
Gross Unrealized Loss		0
Fair Value		56,689
Cash and cash equivalents \| Cash
Amortized Cost	65,584
Gross Unrealized Gains	0
Gross Unrealized Loss	0
Fair Value	65,584
Short-term Investments [Abstract]
Amortized Cost		49,819
Gross Unrealized Gains		0
Gross Unrealized Loss		0
Fair Value		49,819
Cash and cash equivalents \| Money market funds
Amortized Cost	2,284
Gross Unrealized Gains	0
Gross Unrealized Loss	0
Fair Value	2,284
Short-term Investments [Abstract]
Amortized Cost		6,870
Gross Unrealized Gains		0
Gross Unrealized Loss		0
Fair Value		6,870
Cash and cash equivalents \| US Treasury securities
Amortized Cost	39,864
Gross Unrealized Gains	0
Gross Unrealized Loss	(13)
Fair Value	39,851
Short-term Investments [Abstract]
Amortized Cost		39,758
Gross Unrealized Loss		(27)
Fair Value		39,731
Short-term investments
Amortized Cost	90,227
Gross Unrealized Gains	0
Gross Unrealized Loss	(13)
Fair Value	90,214
Short-term Investments [Abstract]
Amortized Cost		79,895
Gross Unrealized Loss		(27)
Fair Value		79,868
Short-term investments \| Term bank deposits
Amortized Cost	50,363
Gross Unrealized Gains	0
Gross Unrealized Loss	0
Fair Value	$ 50,363
Short-term Investments [Abstract]
Amortized Cost		40,137
Gross Unrealized Gains		0
Gross Unrealized Loss		0
Fair Value		$ 40,137

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount of investment in debt security measured at amortized cost (held-to-maturity), investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale) and investment in debt security measured at fair value with change in fair value recognized in net income (trading).

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