Income Taxes - Reconciliation of unrecognized tax benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits, beginning of period	$ 627	$ 938	$ 847
Gross increases - tax positions in current period	0	56	91
Decreases in prior year positions	(150)	(367)	0
Unrecognized tax benefits, end of period	$ 477	$ 627	$ 938