Marketable Securities (Details) - USD ($)	3 Months Ended	12 Months Ended
Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 59,656,000	$ 93,455,000
Gross Unrealized Gains	4,000	2,000
Gross Unrealized Losses	(27,000)	(63,000)
Fair Value	59,633,000	93,394,000
Other-than-temporary unrealized losses	0	0
U.S. Treasury securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,999,000	3,995,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(2,000)	(6,000)
Fair Value	3,997,000	3,989,000
Government-sponsored enterprises
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		11,998,000
Gross Unrealized Gains		0
Gross Unrealized Losses		(7,000)
Fair Value		11,991,000
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	55,657,000	77,462,000
Gross Unrealized Gains	4,000	2,000
Gross Unrealized Losses	(25,000)	(50,000)
Fair Value	$ 55,636,000	$ 77,414,000

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

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

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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.

+ References

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
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-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.2)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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Amount of debt securities categorized neither as held-to-maturity nor trading.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
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-Name Accounting Standards Codification
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-Section 45
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Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section 50
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The amount by which the fair value of an investment in debt and equity securities categorized as Available-for-sale is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 8A
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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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