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Income taxes - Schedule of Valuation Allowances (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Deferred Tax Assets, Valuation Allowance [Roll Forward]      
Beginning at beginning of period $ 125,894 $ 136,418 $ 209,939
Additions to valuation allowance 9,545 3,908 40,180
Reductions to valuation allowance (89,778) (14,432) (113,701)
Balance at the end of the period $ 45,661 $ 125,894 $ 136,418