Fair Value of Financial Instruments - Fair Value Hierarchy for Financial Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair value hierarchy for its financial assets
Transfer of assets between Level 1 to Level 2	$ 0	$ 0
Transfer of assets between Level 2 to Level 1	0	0
Transfer of liabilities between Level 1 to Level 2	0	0
Transfer of liabilities between Level 2 to Level 1	0	0
Fair Value, Measurements, Recurring
Fair value hierarchy for its financial assets
Total assets		8,358
Fair Value, Measurements, Recurring \| Money market funds
Fair value hierarchy for its financial assets
Available-for-sale Securities		84
Fair Value, Measurements, Recurring \| U.S. government-sponsored enterprise bonds
Fair value hierarchy for its financial assets
Available-for-sale Securities		3,767
Fair Value, Measurements, Recurring \| Municipal Bonds
Fair value hierarchy for its financial assets
Available-for-sale Securities		4,027
Fair Value, Measurements, Recurring \| Corporate notes
Fair value hierarchy for its financial assets
Available-for-sale Securities	621	480
Fair Value, Measurements, Recurring \| Level 1
Fair value hierarchy for its financial assets
Total assets		84
Fair Value, Measurements, Recurring \| Level 1 \| Money market funds
Fair value hierarchy for its financial assets
Available-for-sale Securities		84
Fair Value, Measurements, Recurring \| Level 1 \| Corporate notes
Fair value hierarchy for its financial assets
Available-for-sale Securities	$ 621
Fair Value, Measurements, Recurring \| Level 2
Fair value hierarchy for its financial assets
Total assets		8,274
Fair Value, Measurements, Recurring \| Level 2 \| U.S. government-sponsored enterprise bonds
Fair value hierarchy for its financial assets
Available-for-sale Securities		3,767
Fair Value, Measurements, Recurring \| Level 2 \| Municipal Bonds
Fair value hierarchy for its financial assets
Available-for-sale Securities		4,027
Fair Value, Measurements, Recurring \| Level 2 \| Corporate notes
Fair value hierarchy for its financial assets
Available-for-sale Securities		$ 480

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
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-Paragraph 1
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-Name Accounting Standards Codification
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