Amount of cash and cash equivalents restricted as to withdrawal or usage. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits. Cash equivalents include, but are not limited to, 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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-Name Accounting Standards Codification
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-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(2))
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
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Name:	us-gaap_RestrictedCashAndCashEquivalents
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Summary of Significant Accounting Polices - Cash and Cash Equivalents and Restricted Cash and Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash and Cash Equivalents and Restricted Cash
Cash and cash equivalents	$ 15,294	$ 16,369
Restricted cash	113	113
Total	15,407	16,482	$ 23,709
Fair Value of Financial Instruments
Transfers of assets or liabilities between levels	0	0
Money market funds
Cash and Cash Equivalents and Restricted Cash
Cash and cash equivalents	5,956	13,588
Corporate debt securities
Cash and Cash Equivalents and Restricted Cash
Cash and cash equivalents	1,948
Reverse repurchase agreements
Cash and Cash Equivalents and Restricted Cash
Cash and cash equivalents	3,000
Cash
Cash and Cash Equivalents and Restricted Cash
Cash and cash equivalents	$ 4,390	$ 2,781
Minimum \| Government Securities and Obligations \| Repurchase agreements
Fair Value of Financial Instruments
Percentage of investment amount to collateralized deposits value	102.00%