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Shareholders' equity (Tables) | 6 Months Ended
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Jun. 30, 2018
Stockholders' Equity Note [Abstract] |
Common and Preferred Stock Activity | The Common and Preferred Stock table below contains additional information about preferred stock conversions during the six months ended June 30, 2018 and 2017.
| Preferred stock |  | Common stock
Common and Preferred Stock Activity (in shares): | Series A convertible |  | Treasury |  | Outstanding
Beginning balance as of December 31, 2017 | 74,364 |  |  | 7,186,521 |  |  | 87,544,777 |
Conversion of convertible preferred stock | (241 | ) |  | — |  |  | 24,167 |
Issuance for share-based compensation plans | — |  |  | — |  |  | 32,147 |
Ending balance as of June 30, 2018 | 74,123 |  |  | 7,186,521 |  |  | 87,601,091 |
Beginning balance as of December 31, 2016 | 75,625 |  |  | 7,186,521 |  |  | 87,250,897 |
Conversion of convertible preferred stock | (279 | ) |  | — |  |  | 27,904 |
Issuance for share-based compensation plans | — |  |  | — |  |  | 37,164 |
Ending balance as of June 30, 2017 | 75,346 |  |  | 7,186,521 |  |  | 87,315,965 |

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- Definition Tabular disclosure of the change in preferred stock, treasury stock and common stock outstanding. + References No definition available. + Details
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