Investment Securities (Details) Investment Schedule - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Investment [Line Items]
Available-for-sale securities, amortized cost	$ 3,545
Available-for-sale Securities, Gross Unrealized Gain	0	$ 0
Available-for-sale Securities, Gross Unrealized Loss	0	0
Available-for-sale Securities, Debt Maturities, without Single Maturity Date, Amortized Cost Basis	3,545	3,237
Available-for-sale Securities, Debt Maturities, without Single Maturity Date, Fair Value	3,545	3,237
Short-term investments, amortized cost	39,422	37,590
Short-term Investments, Gross Unrealized Holding Gains	1	1
Short-term investments, gross unrealized holding losses	(3)	(4)
Short-term Investments, fair value	39,420	37,587
Deposits [Member]
Investment [Line Items]
Available-for-sale securities, amortized cost	2,545	2,237
Available-for-sale Securities, Gross Unrealized Gain	0	0
Available-for-sale Securities, Gross Unrealized Loss	0	0
Available-for-sale Securities, Debt Maturities, without Single Maturity Date, Fair Value	2,545	2,237
Municipal Bonds [Member]
Investment [Line Items]
Available-for-sale securities, amortized cost	1,000	1,000
Available-for-sale Securities, Gross Unrealized Gain	0	0
Available-for-sale Securities, Gross Unrealized Loss	0	0
Available-for-sale Securities, Debt Maturities, without Single Maturity Date, Fair Value	1,000	1,000
U.S. Treasury securities
Investment [Line Items]
Short-term investments, amortized cost	6,646	5,783
Short-term Investments, Gross Unrealized Holding Gains	0	0
Short-term investments, gross unrealized holding losses	(3)	(4)
Short-term Investments, fair value	6,643	5,779
Certificates of deposit
Investment [Line Items]
Short-term investments, amortized cost	439	690
Short-term Investments, Gross Unrealized Holding Gains	1	1
Short-term investments, gross unrealized holding losses	0	0
Short-term Investments, fair value	440	691
US government debt securities
Investment [Line Items]
Short-term investments, amortized cost	32,337	31,117
Short-term Investments, Gross Unrealized Holding Gains	0	0
Short-term investments, gross unrealized holding losses	0	0
Short-term Investments, fair value	$ 32,337	$ 31,117

Line items represent reporting 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.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
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-Sentence Column A

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
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