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Shareholders' equity (Tables) | 3 Months Ended
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Mar. 31, 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 three months ended March 31, 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 | (133 | ) |  | — |  |  | 13,343 |
Issuance for share-based compensation plans | — |  |  | — |  |  | 17,749 |
Ending balance as of March 31, 2018 | 74,231 |  |  | 7,186,521 |  |  | 87,575,869 |
Beginning balance as of December 31, 2016 | 75,625 |  |  | 7,186,521 |  |  | 87,250,897 |
Conversion of convertible preferred stock | (48 | ) |  | — |  |  | 4,795 |
Issuance for share-based compensation plans | — |  |  | — |  |  | 6,423 |
Ending balance as of March 31, 2017 | 75,577 |  |  | 7,186,521 |  |  | 87,262,115 |

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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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