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Shareholders' equity (Details) - USD ($) $ in Thousands | 6 Months Ended | 47 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Jun. 30, 2015 | Jun. 30, 2014 | Jun. 30, 2015 | Mar. 26, 2015 | Dec. 31, 2014 | Aug. 01, 2011
Class of Stock [Line Items] |  |  |  |  |  |
Common stock, shares authorized (in shares) | 195,000,000 |  | 195,000,000 |  | 195,000,000 |
Stock repurchase program, authorized amount | $ 130,000 |  | $ 130,000 |  |  | $ 60,000
Stock repurchase program, additional authorized amount |  |  |  | $ 70,000 |  |
Repurchase of common stock (in shares) | 2,400,000 |  | 7,186,521 |  |  |
Repurchase of common stock, aggregate purchase price | $ 36,352 |  | $ 86,276 |  |  |
Stock repurchase program, remaining authorized repurchase amount | $ 43,724 |  | $ 43,724 |  |  |
Increase (Decrease) in Stockholders' Equity [Roll Forward] |  |  |  |  |  |
Repurchase of common stock (in shares) | 2,400,000 |  | 7,186,521 |  |  |
Series A Convertible Preferred Stock [Member] |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Repurchase of common stock (in shares) | 0 |  |  |  |  |
Increase (Decrease) in Stockholders' Equity [Roll Forward] |  |  |  |  |  |
Shares, Beginning balance (in shares) | 78,141 | 79,620 |  |  |  |
Repurchase of common stock (in shares) | 0 |  |  |  |  |
Conversion of convertible preferred stock (in shares) | (1,014) | (394) |  |  |  |
Issuance for stock compensation plans (in shares) | 0 | 0 |  |  |  |
Shares, Ending balance (in shares) | 77,127 | 79,226 | 77,127 |  |  |
Treasury Stock [Member] |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Repurchase of common stock (in shares) | 2,400,000 |  |  |  |  |
Increase (Decrease) in Stockholders' Equity [Roll Forward] |  |  |  |  |  |
Shares, Beginning balance (in shares) | 4,786,521 | 4,786,521 |  |  |  |
Repurchase of common stock (in shares) | 2,400,000 |  |  |  |  |
Conversion of convertible preferred stock (in shares) | 0 | 0 |  |  |  |
Issuance for stock compensation plans (in shares) | 0 | 0 |  |  |  |
Shares, Ending balance (in shares) | 7,186,521.000 | 4,786,521 | 7,186,521.000 |  |  |
Common Stock [Member] |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Repurchase of common stock (in shares) | (2,400,000) |  |  |  |  |
Increase (Decrease) in Stockholders' Equity [Roll Forward] |  |  |  |  |  |
Shares, Beginning balance (in shares) | 89,064,582 | 88,710,277 |  |  |  |
Repurchase of common stock (in shares) | (2,400,000) |  |  |  |  |
Conversion of convertible preferred stock (in shares) | 101,327 | 39,455 |  |  |  |
Issuance for stock compensation plans (in shares) | 135,043 | 55,029 |  |  |  |
Shares, Ending balance (in shares) | 86,900,952.000 | 88,804,761 | 86,900,952.000 |  |  |

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