The Company and Summary of Significant Accounting Policies - Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Accumulated other comprehensive income (loss):
Accumulated other comprehensive income (loss) attributable to AXT, Inc.	$ 3,209	$ (4,862)
Accumulated other comprehensive income including noncontrolling interest
Accumulated other comprehensive income (loss):
Unrealized gain ( loss) on investments, net	3	(3)
Cumulative translation adjustment	3,601	(4,842)
Accumulated other comprehensive income (loss) attributable to AXT, Inc.	3,604	(4,845)
Accumulated other comprehensive income attributable to noncontrolling interest
Accumulated other comprehensive income (loss):
Less: Cumulative translation adjustment attributable to noncontrolling interests	$ 395	$ 17