Fair value of financial instruments and marketable securities - Marketable securities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Marketable securities on the balance sheet
Total Marketable securities, Less Than 12 Months	$ 72,318	$ 69,377
Total Marketable securities, More Than 12 Months	0	10,077
Commercial paper
Marketable securities on the balance sheet
Total Marketable securities, Less Than 12 Months	26,328	13,827
Total Marketable securities, More Than 12 Months	0	0
Corporate debt securities
Marketable securities on the balance sheet
Total Marketable securities, Less Than 12 Months	45,990	55,550
Total Marketable securities, More Than 12 Months	$ 0	$ 10,077

Fair value of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), maturing in next fiscal year following latest fiscal year. Excludes interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27198-111563

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 320
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=75019621&loc=d3e62586-112803

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant