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Shareholders' equity (Tables) | 6 Months Ended
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Jun. 30, 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 the six months ended June 30, 2016 and 2015.
Common and Preferred Stock Activity: | Preferred stock |  | Common stock
(in shares) | Series A convertible |  | Treasury | Outstanding
Beginning balance as of December 31, 2015 | 76,539 |  |  | 7,186,521 |  | 87,038,050 |
Repurchase of common stock | — |  |  | — |  | — |
Conversion of convertible preferred stock | (161 | ) |  | — |  | 16,109 |
Issuance for share-based compensation plans | — |  |  | — |  | 21,428 |
Ending balance as of June 30, 2016 | 76,378 |  |  | 7,186,521 |  | 87,075,587 |
Beginning 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 | (1,014 | ) |  | — |  | 101,327 |
Issuance for share-based compensation plans | — |  |  | — |  | 135,043 |
Ending balance as of June 30, 2015 | 77,127 |  |  | 7,186,521 |  | 86,900,952 |

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