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Stockholders' equity - Additional Information (Details) - $ / shares | Dec. 31, 2018 | Dec. 31, 2017
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Equity [Abstract] |  |
Common stock, shares authorized (in shares) | 150,000,000 | 150,000,000
Common stock, par value (in dollars per share) | $ 0.01 | $ 0.01
Preferred stock, shares authorized (in shares) | 40,000,000 |
Preferred stock, par value (in dollars per share) | $ 1 |
Unissued common stock reserved for stock-based incentive plans and convertible debt (in shares) | 34,400,000 |

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