Marketable Securities (Details) - USD ($)	6 Months Ended	12 Months Ended
Jun. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 127,113,000	$ 200,966,000
Gross Unrealized Gains	5,000	56,000
Gross Unrealized Losses	(85,000)	(138,000)
Fair Value	127,033,000	200,884,000
Other-than-temporary unrealized losses	0	0
U.S. Treasury securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	13,802,000	4,826,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(20,000)	(1,000)
Fair Value	13,782,000	4,825,000
Government-sponsored enterprises
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	26,159,000	29,764,000
Gross Unrealized Gains	0	5,000
Gross Unrealized Losses	(30,000)	(10,000)
Fair Value	26,129,000	29,759,000
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	87,152,000	166,376,000
Gross Unrealized Gains	5,000	51,000
Gross Unrealized Losses	(35,000)	(127,000)
Fair Value	$ 87,122,000	$ 166,300,000

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=73527792&loc=d3e27161-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section S99
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-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
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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
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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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-Topic 320
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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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