Income taxes - Assessing realizability of deferred tax assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Federal
Net operating loss carryforwards
The amount of gross unrealized net operating loss carryforwards	$ 124,637	$ 82,461
State
Net operating loss carryforwards
The amount of gross unrealized net operating loss carryforwards	121,091	73,934
Foreign
Net operating loss carryforwards
The amount of gross unrealized net operating loss carryforwards	$ 11,642	$ 10,811