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Income taxes (Summary of valuation allowance) (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Deferred Tax Assets, Valuation Allowance [Roll Forward]    
Valuation allowance at beginning of period $ 104,858 $ 127,361
Decrease of allowance to income tax provision (12,560) (22,503)
Valuation allowance at end of period $ 92,298 $ 104,858