Marketable Securities - Gross unrealized gains/losses (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Marketable securities
Estimated fair value	$ 26,358	$ 45,236
Due in one year or less
Marketable securities
Amortized cost	(26,367)	(45,266)
Gross unrealized gains	0	0
Gross unrealized losses	(9)	(30)
Estimated fair value	26,358	45,236
Corporate notes and bonds
Marketable securities
Estimated fair value	13,801	26,278
Corporate notes and bonds \| Due in one year or less
Marketable securities
Amortized cost	(13,809)	(26,303)
Gross unrealized gains	0	0
Gross unrealized losses	(8)	(25)
Estimated fair value	13,801	26,278
U.S. government and agency securities
Marketable securities
Estimated fair value	3,483	11,976
U.S. government and agency securities \| Due in one year or less
Marketable securities
Amortized cost	(3,484)	(11,981)
Gross unrealized gains	0	0
Gross unrealized losses	(1)	(5)
Estimated fair value	3,483	11,976
Commercial paper
Marketable securities
Estimated fair value	9,074	6,982
Commercial paper \| Due in one year or less
Marketable securities
Amortized cost	(9,074)	(6,982)
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Estimated fair value	$ 9,074	$ 6,982

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562

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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://www.xbrl.org/2003/role/presentationRef
-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

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

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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

Reference 1: 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)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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