Cash and Cash Equivalents and Investments (Details 1) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Cash and Cash Equivalents
Fair Value
Amortized Cost	$ 15,044	$ 61,661
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	15,044	61,661
U.S. Treasury Securities
Fair Value
Amortized Cost	174,262	222,316
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(180)	(168)
Estimated Fair Value	174,082	222,148
Corporate Debt Securities
Fair Value
Amortized Cost	20,601	27,033
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(38)	(54)
Estimated Fair Value	20,563	26,979
Total Short-term Investments
Fair Value
Amortized Cost	194,863	249,349
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(218)	(222)
Estimated Fair Value	194,645	249,127
Total Cash and Cash Equivalents and Investments
Fair Value
Amortized Cost	209,907	311,010
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(218)	(222)
Estimated Fair Value	$ 209,689	$ 310,788

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as 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)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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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 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.

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