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Shareholders' equity (Tables) | 12 Months Ended
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Dec. 31, 2017
Stockholders' Equity Note [Abstract] |
Schedule of Common and Preferred Stock Activity | The Common and Preferred Stock Activity table below contains additional information about preferred stock conversions during 2017, 2016 and 2015:
Common and Preferred Stock Activity: | Preferred stock |  | Common stock
(in shares) | Series A Convertible |  | Treasury |  | Outstanding
Balance as of December 31, 2014 | 78,141 |  |  | 4,786,521 |  |  | 89,064,582 |
Repurchase of common stock | — |  |  | 2,400,000 |  |  | (2,400,000 | )
Conversion of convertible preferred stock | (1,602 | ) |  | — |  |  | 160,162 |
Issuance for share-based compensation plans | — |  |  | — |  |  | 213,306 |
Beginning balance as of December 31, 2015 | 76,539 |  |  | 7,186,521 |  |  | 87,038,050 |
Repurchase of common stock | — |  |  | — |  |  | — |
Conversion of convertible preferred stock | (914 | ) |  | — |  |  | 91,362 |
Issuance for share-based compensation plans | — |  |  | — |  |  | 121,485 |
Beginning balance as of December 31, 2016 | 75,625 |  |  | 7,186,521 |  |  | 87,250,897 |
Repurchase of common stock | — |  |  | — |  |  | — |
Conversion of convertible preferred stock | (1,261 | ) |  | — |  |  | 126,098 |
Issuance for share-based compensation plans | — |  |  | — |  |  | 167,782 |
Ending balance as of December 31, 2017 | 74,364 |  |  | 7,186,521 |  |  | 87,544,777 |

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