Fair Value Measurements (Details) - Recurring - Money market funds, US Government Securities and Agency Investments - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Total
Fair Value Measurements
Money market funds, U.S. Government Securities and Agency Investments	$ 129,620	$ 116,433
Level 1
Fair Value Measurements
Money market funds, U.S. Government Securities and Agency Investments	$ 129,620	$ 116,433

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.

No definition available.

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant