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Income Taxes - Changes in valuation allowance (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Changes in valuation allowance for deferred tax assets:      
Valuation allowance at beginning of year $ 15,542 $ 25,579 $ 21,726
Increases (decreases) recorded to income tax provision 2,882 (10,037) 3,853
Valuation allowance at end of year $ 18,424 $ 15,542 $ 25,579