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Capital structure - Narrative (Details) - shares | Dec. 31, 2017 | Dec. 31, 2016
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Capital structure |  |
Common stock, authorized shares (in shares) | 125,000,000 | 125,000,000
Common stock |  |
Capital structure |  |
Common stock, authorized shares (in shares) | 125,000,000 |

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- Definition The maximum number of common shares permitted to be issued by an entity's charter and bylaws. + 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(29)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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