Investments (Details) - Short-term investments [Member] - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 13,490	$ 6,452
Gross unrealized gains
Gross unrealized losses	(1)	(1)
Fair value	13,489	6,451
U.S. Treasury securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized cost	1,499
Gross unrealized gains
Gross unrealized losses
Fair value	1,499
Commercial paper [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized cost	11,730	4,994
Gross unrealized gains
Gross unrealized losses	(1)	(1)
Fair value	11,729	4,993
Corporate notes / bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized cost	261	1,458
Gross unrealized gains
Gross unrealized losses
Fair value	$ 261	$ 1,458

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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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

+ References

No definition available.

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This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

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Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

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Line items represent financial 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.

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