v3.8.0.1
Income taxes - Changes in valuation allowance (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Changes in the valuation allowance      
Balance at the beginning of the year $ 36,331 $ 19,548 $ 12,470
Additions charged to operations 16,527 16,783 7,078
Effect of U.S. tax reform law changes (17,868)    
Decrease credited to operations 0 0 0
Balance at the end of the year $ 34,990 $ 36,331 $ 19,548
Period of cumulative results for determination of releasing valuation allowance 3 years