Income Taxes - Valuation allowances, uncertain tax positions and accrued interest and penalties (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Valuation allowance
Increase (decrease) in valuation allowance	$ 2,700	$ 2,100
Interest and penalties
Accrued interest or penalties for income tax liabilities	100	200	$ 100
Change in reserve for uncertain tax benefits
Reserve at beginning of year	2,923	427	107
Increases related to prior year tax positions	277	2,500	427
Decreases related to prior year tax positions	(37)
Lapse of statute of limitations		(4)	(107)
Settlement	(2,663)
Total	500	$ 2,923	427
Uncertain tax positions
Uncertain tax positions that would impact of effective tax rate	500
Unrecognized tax benefits estimated decrease	$ 500
Belgium
Uncertain tax positions
Total assessment			$ 0	$ 0