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Shareholders’ equity (Details) (USD $) In Thousands, except Share data, unless otherwise specified | 1 Months Ended | 6 Months Ended | 23 Months Ended |
--- | --- | --- | --- | ---
Aug. 31, 2011 | Jun. 30, 2013 | Jun. 30, 2012 | Jun. 30, 2013 | Dec. 31, 2012
Increase (Decrease) in Stockholders' Equity [Roll Forward] |  |  |  |  |
Common stock, shares authorized (in shares) |  | 195,000,000 |  | 195,000,000 | 195,000,000
Stock repurchase program, Authorized Amount | $ 60,000 |  |  |  |
Number of shares Treasury stock acquired (in shares) |  |  | 400,000 | 4,786,521 |
Treasury stock acquired, Aggregate purchase price |  |  |  | 49,924 |
Stock repurchase program, Remaining authorized repurchase amount |  | $ 10,076 |  |  |
Series A Convertible Preferred Stock [Member] |  |  |  |  |
Increase (Decrease) in Stockholders' Equity [Roll Forward] |  |  |  |  |
Shares, Beginning balance |  | 80,283 |  |  |
Repurchase of common stock (in shares) |  | 0 |  |  |
Conversion of convertible preferred stock (in shares) |  | (199) |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period |  | 0 |  |  |
Shares, Ending balance |  | 80,084 |  | 80,084 |
Treasury Stock [Member] |  |  |  |  |
Increase (Decrease) in Stockholders' Equity [Roll Forward] |  |  |  |  |
Shares, Beginning balance |  | 4,786,521 |  |  |
Repurchase of common stock (in shares) |  | 0 |  |  |
Conversion of convertible preferred stock (in shares) |  | 0 |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period |  | 0 |  |  |
Shares, Ending balance |  | 4,786,521 |  | 4,786,521 |
Common Stock [Member] |  |  |  |  |
Increase (Decrease) in Stockholders' Equity [Roll Forward] |  |  |  |  |
Shares, Beginning balance |  | 88,548,637 |  |  |
Repurchase of common stock (in shares) |  | 0 |  |  |
Conversion of convertible preferred stock (in shares) |  | 19,864 |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period |  | 34,208 |  |  |
Shares, Ending balance |  | 88,602,709 |  | 88,602,709 |

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