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Shareholders' equity (Tables) | 12 Months Ended
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Dec. 31, 2016
Stockholders' Equity Note [Abstract] |
Common and Preferred Stock Activity | The Common and Preferred Stock Activity table below contains additional information about preferred stock conversions during 2016, 2015 and 2014:
Common and Preferred Stock Activity: | Preferred stock | Common stock
(in shares) | Series A Convertible | Treasury | Outstanding
Balance as of December 31, 2013 | 79,620 |  | 4,786,521 |  | 88,710,277 |
Conversion of convertible preferred stock | (1,479 | ) | — |  | 147,952 |
Issuance for share-based compensation plans | — |  | — |  | 206,353 |
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 |
Balance as of December 31, 2015 | 76,539 |  | 7,186,521 |  | 87,038,050 |
Conversion of convertible preferred stock | (914 | ) | — |  | 91,362 |
Issuance for share-based compensation plans | — |  | — |  | 121,485 |
Balance as of December 31, 2016 | 75,625 |  | 7,186,521 |  | 87,250,897 |

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