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Cash, cash equivalents and short-term investments - Narrative (Details) | 3 Months Ended | 6 Months Ended |  |
--- | --- | --- | --- | ---
Jun. 30, 2018 USD ($) security | Jun. 30, 2017 USD ($) | Jun. 30, 2018 USD ($) security | Jun. 30, 2017 USD ($) | Dec. 31, 2017 USD ($) security | Dec. 31, 2016 USD ($) security
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |  |
Debt securities in an unrealized position for less than 12 months (in securities) | security | 5 |  | 5 |  | 10 | 21
Aggregate fair value of securities in an unrealized loss position for less than 12 months | $ 10,400,000 |  | $ 10,400,000 |  | $ 22,900,000 | $ 46,600,000
Number of investments in a continuous unrealized loss position (in securities) | security | 5 |  | 5 |  |  |
Securities held | $ 53,637,000 |  | $ 53,637,000 |  | 67,230,000 | 114,448,000
Debt securities |  |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |  |
Securities held | 7,900,000 |  | 7,900,000 |  | 12,000,000 | 11,500,000
Short-term investments |  |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |  |
Securities held | 29,036,000 |  | 29,036,000 |  | $ 51,527,000 | $ 70,518,000
Realized gains on investments | 0 | $ 0 | 0 | $ 0 |  |
Realized losses on investments | $ 0 | $ 0 | $ 0 | $ 0 |  |

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- Definition This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for less than twelve months. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563 + Details
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