Income taxes - Assessing realizability of deferred tax assets (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Federal
Net operating loss carryforwards
The amount of gross unrealized net operating loss carryforwards	$ 170,907	$ 164,373
Operating loss carryforwards to be carried forward indefinitely	87,162
State
Net operating loss carryforwards
The amount of gross unrealized net operating loss carryforwards	181,488	170,831
Foreign
Net operating loss carryforwards
The amount of gross unrealized net operating loss carryforwards	$ 11,710	$ 11,671