Income Taxes - Reconciliation of unrecognized tax benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits, beginning of period	$ 847	$ 336	$ 170
Gross increases - tax positions in current period	91	130	166
Gross increases - tax positions in prior period	0	381	0
Unrecognized tax benefits, end of period	$ 938	$ 847	$ 336