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Income taxes - Components of Pre-tax Book Income (Loss) (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Income Tax Disclosure [Abstract]      
U.S. $ 26,823 $ (86,545) $ (62,203)
Foreign 28,548 (164,320) 21,081
Income (loss) before income taxes and equity in earnings of joint ventures $ 55,371 $ (250,865) $ (41,122)