Short-term investments - Unrealized Gains And Losses Related To Cash, Cash Equivalents, And Short-Term Investments Designated As Available-For-Sale (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Total adjusted cost of available-for-sale securities	$ 211,574	$ 122,101
Gross Unrealized Gain	178	182
Gross Unrealized Loss	(948)	(395)
Fair Value	210,804	121,888
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Total adjusted cost of available-for-sale securities	170,921	120,341
Gross Unrealized Gain	177	182
Gross Unrealized Loss	(948)	(395)
Fair Value	170,150	120,128
U.S. treasuries and agencies
Debt Securities, Available-for-sale [Line Items]
Total adjusted cost of available-for-sale securities	39,353
Gross Unrealized Gain	1
Gross Unrealized Loss	0
Fair Value	39,354
Time deposits
Debt Securities, Available-for-sale [Line Items]
Total adjusted cost of available-for-sale securities	1,300	1,760
Gross Unrealized Gain	0	0
Gross Unrealized Loss	0	0
Fair Value	$ 1,300	$ 1,760

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-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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- Definition

Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-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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- Definition

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

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

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- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562

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-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

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