Marketable Securities (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Marketable Securities, Interest Income	$ 0	$ 0
Amortized Cost	8,320	25,712
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	0	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(1)	0
Fair Value	8,319	25,712
Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,002	3,864
Fair Value	1,001	3,864
Money market mutual funds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,318	21,848
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	0	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	0
Fair Value	7,318	21,848
United States treasuries
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,002
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(1)
Fair Value	$ 1,001
Certificates of deposit
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		3,864
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax		0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax		0
Fair Value		$ 3,864

This item represents the cost of debt 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.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.2)
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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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