Fair Value Measurements (Details 1) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Fair Value
Cash and Cash Equivalents	$ 15,044	$ 61,661
Short-term Investments	194,645	249,127
Total Cash and Cash Equivalents and Investments	209,689	310,788
Fair Value, Level 1
Fair Value
Cash and Cash Equivalents	15,044	61,661
Short-term Investments	174,082	222,148
Total Cash and Cash Equivalents and Investments	189,126	283,809
Fair Value, Level 2
Fair Value
Cash and Cash Equivalents	0	0
Short-term Investments	20,563	26,979
Total Cash and Cash Equivalents and Investments	20,563	26,979
Fair Value, Level 3
Fair Value
Cash and Cash Equivalents	0	0
Short-term Investments	0	0
Total Cash and Cash Equivalents and Investments	$ 0	$ 0

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

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- Definition

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.

+ References

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

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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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