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Financial Instruments (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Financial Instruments
Transfers between fair value measurement levels	$ 0		$ 0
Cash and cash equivalents:
Cash and cash equivalents	$ 17,030	$ 16,369	$ 31,795
Government Securities and Obligations | Reverse Repurchase Agreement | Minimum
Financial Instruments
Percentage of investment amount to collateralized deposits value	102.00%
Recurring
Assets:
Total assets	$ 13,623	13,588
Recurring | Money market funds
Cash and cash equivalents:
Cash and cash equivalents	5,623	13,588
Recurring | U.S. reverse repurchase agreements
Cash and cash equivalents:
Cash and cash equivalents	8,000
Recurring | Level 1
Assets:
Total assets	5,623	13,588
Recurring | Level 1 | Money market funds
Cash and cash equivalents:
Cash and cash equivalents	5,623	$ 13,588
Recurring | Level 2
Assets:
Total assets	8,000
Recurring | Level 2 | U.S. reverse repurchase agreements
Cash and cash equivalents:
Cash and cash equivalents	$ 8,000

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82887183&loc=d3e6676-107765

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-Name Accounting Standards Codification
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-Section 45
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-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.1)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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