Stockholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended	60 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2015	Dec. 31, 2020	Oct. 27, 2014
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	$ 207,969	$ 192,762
Common stock options exercised	704	(441)
Stock-based compensation	816	643
Formation of new subsidiary with noncontrolling interests	576
Sale of common stock to employees in connection with the reorganization	538
Purchase of subsidiary shares from noncontrolling interest	(3,730)
Net income (loss)	3,736
Net income (loss)	3,765	217
Other comprehensive loss	(147)	(1,873)
Balance, end of period	210,462	192,190		$ 207,969
Reclassification adjustment from AOCI	0	0
Stock repurchase program, authorized amount					$ 5,000
Shares repurchased (in shares)			908,000	0
Average price of shares repurchased (in dollars per share)			$ 2.52
Total purchase price			$ 2,300
Stock repurchase program remaining authorized repurchase amount	2,700
Preferred Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	3,532	3,532
Balance, end of period	3,532	3,532		$ 3,532
Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	42	41
Balance, end of period	42	41		42
Additional Paid-In Capital
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	230,381	236,957
Common stock options exercised	704	(441)
Stock-based compensation	816	643
Formation of new subsidiary with noncontrolling interests	(131)
Sale of common stock to employees in connection with the reorganization	538
Purchase of subsidiary shares from noncontrolling interest	(2,691)
Balance, end of period	229,617	238,041		230,381
Accumulated Deficit
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	(44,545)	(47,783)
Net income (loss)	3,425
Net income (loss)		(178)
Balance, end of period	(41,120)	(47,961)		(44,545)
Accumulated Other Comprehensive Income (Loss)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	3,209	(4,862)
Other comprehensive loss	(131)	(1,775)
Balance, end of period	3,078	(6,637)		3,209
AXT, Inc. Stockholders' Equity
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	192,619	187,885
Common stock options exercised	704	(441)
Stock-based compensation	816	643
Formation of new subsidiary with noncontrolling interests	(131)
Sale of common stock to employees in connection with the reorganization	538
Purchase of subsidiary shares from noncontrolling interest	(2,691)
Net income (loss)	3,425
Net income (loss)		(178)
Other comprehensive loss	(131)	(1,775)
Balance, end of period	195,149	187,016		192,619
Noncontrolling Interests
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	15,350	4,877
Formation of new subsidiary with noncontrolling interests	707
Purchase of subsidiary shares from noncontrolling interest	(1,039)
Net income (loss)	311
Net income (loss)		395
Other comprehensive loss	(16)	(98)
Balance, end of period	$ 15,313	$ 5,174		$ 15,350