Significant Accounting Policies - Cash, Cash Equivalents and Short-term Deposits (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Cash and Cash Equivalents
Significant Accounting Policies [Line Items]
Amortized Cost	$ 2,594	$ 6,870
Gross Unrealized Gains	0	0
Gross Unrealized Loss	0	0
Fair value	2,594	6,870
Cash and Cash Equivalents \| Money Market Funds
Significant Accounting Policies [Line Items]
Amortized Cost	2,594	6,870
Gross Unrealized Gains	0	0
Gross Unrealized Loss	0	0
Fair value	2,594	6,870
Marketable Securities
Significant Accounting Policies [Line Items]
Amortized Cost	39,776
Gross Unrealized Loss	(6)
Fair value	39,770
Marketable Securities \| US Treasury Securities
Significant Accounting Policies [Line Items]
Amortized Cost	39,776	39,758
Gross Unrealized Loss	(6)	(27)
Fair value	39,770	39,731
Short-term Deposits
Significant Accounting Policies [Line Items]
Amortized Cost	70,438	40,137
Gross Unrealized Loss	0	0
Fair value	70,438	40,137
Short-term Deposits \| Term Bank Deposits
Significant Accounting Policies [Line Items]
Amortized Cost	70,438	40,137
Gross Unrealized Gains	0	0
Gross Unrealized Loss	0	0
Fair value	$ 70,438	$ 40,137

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

No definition available.

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- Definition

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

+ References

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

+ Details

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- Definition

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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- Definition

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

No definition available.

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- Definition

[Line Items] for Significant Accounting Policies [Table]

+ References

No definition available.

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