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Shareholders' equity (Details) - USD ($) | 12 Months Ended | 33 Months Ended | 84 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2017 | Dec. 31, 2017 | Mar. 31, 2015 | Dec. 31, 2011 | Dec. 31, 2008
Class of Stock [Line Items] |  |  |  |  |  |  |  |
Common stock, shares authorized (shares) | 195,000,000 | 195,000,000 |  | 195,000,000 | 195,000,000 |  |  |
Common stock, par value (in dollars per share) | $ 0.01 | $ 0.01 |  | $ 0.01 | $ 0.01 |  |  |
Common stock, shares issued (shares) | 94,731,298 | 94,437,418 |  | 94,731,298 | 94,731,298 |  |  |
Common stock, shares outstanding (shares) | 87,544,777 | 87,250,897 |  | 87,544,777 | 87,544,777 |  |  |
Preferred stock, shares authorized (shares) | 5,000,000 | 5,000,000 |  | 5,000,000 | 5,000,000 |  |  |
Preferred stock, par value (in dollars per share) |  | $ 0.01 |  |  |  |  |  |
Preferred stock, shares issued (shares) |  |  |  |  |  |  |  | 160,000
Conversion of convertible preferred stock (shares) | 100 |  |  | 100 | 100 |  |  |
Liquidation preference (in dollars per share) | $ 0.01 |  |  | $ 0.01 | $ 0.01 |  |  |
Authorized repurchase amount | $ 130,000,000 |  |  | $ 130,000,000 | $ 130,000,000 |  | $ 60,000,000 |
Additional repurchase amount |  |  |  |  |  | $ 70,000,000 |  |
Treasury stock acquired, aggregate purchase price |  |  | $ 36,352,000 |  | 86,276,000 |  |  |
Remaining authorized repurchase amount | $ 43,724,000 |  |  | $ 43,724,000 | $ 43,724,000 |  |  |
Increase (Decrease) in Stockholders' Equity [Roll Forward] |  |  |  |  |  |  |  |
Repurchase of common stock (shares) |  |  |  | 0 | 7,186,521 |  |  |
Series A Convertible Preferred Stock [Member] |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |
Preferred stock, par value (in dollars per share) | $ 0.01 |  |  | $ 0.01 | $ 0.01 |  |  |
Preferred stock, shares outstanding (shares) | 74,364 | 75,625 |  | 74,364 | 74,364 |  |  |
Increase (Decrease) in Stockholders' Equity [Roll Forward] |  |  |  |  |  |  |  |
Beginning balance (shares) | 75,625 | 76,539 | 78,141 |  |  |  |  |
Repurchase of common stock (shares) | 0 | 0 | 0 |  |  |  |  |
Conversion of convertible preferred stock (shares) | (1,261) | (914) | (1,602) |  |  |  |  |
Issuance for share-based compensation plans (shares) | 0 | 0 | 0 |  |  |  |  |
Ending balance (shares) | 74,364 | 75,625 | 76,539 | 74,364 | 74,364 |  |  |
Treasury [Member] |  |  |  |  |  |  |  |
Increase (Decrease) in Stockholders' Equity [Roll Forward] |  |  |  |  |  |  |  |
Beginning balance (shares) | 7,186,521 | 7,186,521 | 4,786,521 |  |  |  |  |
Repurchase of common stock (shares) | 0 | 0 | 2,400,000 |  |  |  |  |
Conversion of convertible preferred stock (shares) | 0 | 0 | 0 |  |  |  |  |
Issuance for share-based compensation plans (shares) | 0 | 0 | 0 |  |  |  |  |
Ending balance (shares) | 7,186,521 | 7,186,521 | 7,186,521 | 7,186,521 | 7,186,521 |  |  |
Common stock [Member] |  |  |  |  |  |  |  |
Increase (Decrease) in Stockholders' Equity [Roll Forward] |  |  |  |  |  |  |  |
Beginning balance (shares) | 87,250,897 | 87,038,050 | 89,064,582 |  |  |  |  |
Repurchase of common stock (shares) | 0 | 0 | 2,400,000 |  |  |  |  |
Conversion of convertible preferred stock (shares) | 126,098 | 91,362 | 160,162 |  |  |  |  |
Issuance for share-based compensation plans (shares) | 167,782 | 121,485 | 213,306 |  |  |  |  |
Ending balance (shares) | 87,544,777 | 87,250,897 | 87,038,050 | 87,544,777 | 87,544,777 |  |  |

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