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Shareholders' equity (Details) - USD ($) | 6 Months Ended | 39 Months Ended | 90 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | --- | ---
Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2018 | Dec. 31, 2017 | Mar. 31, 2015 | Dec. 31, 2011 | Dec. 31, 2008
Class of Stock [Line Items] |  |  |  |  |  |  |  |
Common stock, shares authorized (in 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 (in shares) | 94,787,612 |  | 94,787,612 | 94,787,612 | 94,731,298 |  |  |
Common stock, shares, outstanding (in shares) | 87,601,091 |  | 87,601,091 | 87,601,091 | 87,544,777 |  |  |
Preferred stock, shares authorized (in shares) | 5,000,000 |  | 5,000,000 | 5,000,000 | 5,000,000 |  |  |
Preferred stock par value (in dollars per share) | $ 0.01 |  | $ 0.01 | $ 0.01 | $ 0.01 |  |  |
Preferred shares issued (in shares) |  |  |  |  |  |  |  | 160,000
Number of shares issued for each share of preferred stock (in shares) | 100 |  | 100 | 100 |  |  |  |
Stock repurchase program, authorized amount | $ 130,000,000 |  | $ 130,000,000 | $ 130,000,000 |  | $ 70,000,000 | $ 60,000,000 |
Treasury stock, shares, acquired (in shares) |  |  | 0 | 7,186,521 |  |  |  |
Treasury stock, value, acquired, cost method |  |  |  | $ 86,300,000 |  |  |  |
Stock repurchase program, remaining authorized repurchase amount | $ 43,700,000 |  | $ 43,700,000 | $ 43,700,000 |  |  |  |
Increase (Decrease) in Stockholders' Equity [Roll Forward] |  |  |  |  |  |  |  |
Shares, Ending balance (in shares) |  | 87,315,965 |  |  |  |  |  |
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 (in shares) | 74,123 |  | 74,123 | 74,123 | 74,364,000 |  |  |
Increase (Decrease) in Stockholders' Equity [Roll Forward] |  |  |  |  |  |  |  |
Shares, Beginning balance (in shares) | 74,364 | 75,625 |  |  |  |  |  |
Conversion of convertible preferred stock (in shares) | (241) | (279) |  |  |  |  |  |
Issuance for stock compensation plans (in shares) | 0 | 0 |  |  |  |  |  |
Shares, Ending balance (in shares) | 74,123.000 | 75,346 | 74,123.000 | 74,123.000 |  |  |  |
Treasury Stock [Member] |  |  |  |  |  |  |  |
Increase (Decrease) in Stockholders' Equity [Roll Forward] |  |  |  |  |  |  |  |
Shares, Beginning balance (in shares) | 7,186,521 | 7,186,521 |  |  |  |  |  |
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 | 7,186,521 | 7,186,521.000 | 7,186,521.000 |  |  |  |
Common Stock [Member] |  |  |  |  |  |  |  |
Increase (Decrease) in Stockholders' Equity [Roll Forward] |  |  |  |  |  |  |  |
Shares, Beginning balance (in shares) | 87,544,777 | 87,250,897 |  |  |  |  |  |
Conversion of convertible preferred stock (in shares) | 24,167 | 27,904 |  |  |  |  |  |
Issuance for stock compensation plans (in shares) | 32,147 | 37,164 |  |  |  |  |  |
Shares, Ending balance (in shares) | 87,601,091.000 |  | 87,601,091.000 | 87,601,091.000 |  |  |  |

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