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Cash, cash equivalents and short-term investments - Summary (Details) - USD ($) $ in Thousands | Jun. 30, 2018 | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |  |
Amortized Cost | $ 53,637 | $ 67,230 | $ 114,448
Gross Unrealized Gains | 0 | 0 | 3
Gross Unrealized Losses | (9) | (27) | (63)
Fair Value | 53,628 | 67,203 | 114,388
Cash and money market funds |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Amortized Cost | 24,601 | 15,703 | 43,930
Gross Unrealized Gains | 0 | 0 | 0
Gross Unrealized Losses | 0 | 0 | 0
Fair Value | 24,601 | 15,703 | 43,930
Corporate debt securities due in one year or less |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Amortized Cost | 21,054 | 38,053 |
Gross Unrealized Gains | 0 | 0 |
Gross Unrealized Losses | (8) | (21) |
Fair Value | 21,046 | 38,032 |
U.S. government securities due In one year or less |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Amortized Cost | 7,982 | 13,474 |
Gross Unrealized Gains | 0 | 0 |
Gross Unrealized Losses | (1) | (6) |
Fair Value | 7,981 | 13,468 |
Cash and cash equivalents |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Amortized Cost | 24,601 | 15,703 | 43,930
Gross Unrealized Gains | 0 | 0 | 0
Gross Unrealized Losses | 0 | 0 | 0
Fair Value | 24,601 | 15,703 | 43,930
Short-term investments |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Amortized Cost | 29,036 | 51,527 | 70,518
Gross Unrealized Gains | 0 | 0 | 3
Gross Unrealized Losses | (9) | (27) | (63)
Fair Value | $ 29,027 | $ 51,500 | $ 70,458

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