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Stockholders' equity, Preferred stock and Common stock (Details) - $ / shares | 12 Months Ended |  |  |
--- | --- | --- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Preferred stock [Abstract] |  |  |  |
Preferred stock, shares authorized (in shares) | 15,000,000 | 15,000,000 |  |
Preferred stock, par value (in dollars per share) | $ 0.001 | $ 0.001 |  |
Common stock [Abstract] |  |  |  |
Common stock, par value (in dollars per share) | $ 0.001 | $ 0.001 | $ 0.001 | $ 0.001
Common stock, shares authorized (in shares) | 200,000,000 | 200,000,000 |  |
Common stock, voting rights | 1 |  |  |

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- Definition Description of voting rights of common stock. Includes eligibility to vote and votes per share owned. Include also, if any, unusual voting rights. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21475-112644 + Details
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- Definition Face amount or stated value per share of preferred stock nonredeemable or redeemable solely at the option of the issuer. + 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(28)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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