Stockholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						9 Months Ended		12 Months Ended	60 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2015	Dec. 31, 2020	Oct. 27, 2014
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	$ 217,262	$ 210,462	$ 207,969	$ 194,054	$ 192,190	$ 192,762	$ 207,969	$ 192,762
Common stock options exercised	468	420	704	624	72	441
Stock-based compensation	1,639	975	816	648	641	643
Formation of new subsidiary with noncontrolling interests		575	576	396
Sale of common stock to employees in connection with the reorganization			538
Purchase of subsidiary shares from noncontrolling interest			(3,730)
Transfer of subsidiary with redeemable noncontrolling interest		(1,241)
Net income (loss)	4,129	4,479	3,736	1,481
Net income (loss)	4,496			1,481	959	217	12,876	2,657
Net dividend declared by joint ventures					(89)
Other comprehensive income (loss)	56	1,592	(147)	4,689	281	(1,873)
Balance, end of period	223,554	217,262	210,462	201,892	194,054	192,190	223,554	201,892		$ 207,969
Reclassification adjustment from AOCI	0			0			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						2,700
Preferred Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	3,532	3,532	3,532	3,532	3,532	3,532	3,532	3,532
Balance, end of period	3,532	3,532	3,532	3,532	3,532	3,532	3,532	3,532		$ 3,532
Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	42	42	42	41	41	41	42	41
Common stock options exercised	1
Balance, end of period	43	42	42	41	41	41	43	41		42
Additional Paid-In Capital
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	228,411	229,617	230,381	238,754	238,041	236,957	230,381	236,957
Common stock options exercised	467	420	704	624	72	441
Stock-based compensation	1,639	975	816	648	641	643
Formation of new subsidiary with noncontrolling interests		(131)	(131)	396
Sale of common stock to employees in connection with the reorganization			538
Purchase of subsidiary shares from noncontrolling interest			(2,691)
Transfer of subsidiary with noncontrolling interest		(1,229)
Transfer of subsidiary with redeemable noncontrolling interest		(1,241)
Balance, end of period	230,517	228,411	229,617	240,422	238,754	238,041	230,517	240,422		230,381
Accumulated Deficit
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	(36,735)	(41,120)	(44,545)	(47,600)	(47,961)	(47,783)	(44,545)	(47,783)
Net income (loss)	3,800	4,385	3,425	991
Net income (loss)					361	(178)
Balance, end of period	(32,935)	(36,735)	(41,120)	(46,609)	(47,600)	(47,961)	(32,935)	(46,609)		(44,545)
Accumulated Other Comprehensive Income (Loss)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	4,533	3,078	3,209	(6,365)	(6,637)	(4,862)	3,209	(4,862)
Other comprehensive income (loss)	50	1,455	(131)	4,373	272	(1,775)
Balance, end of period	4,583	4,533	3,078	(1,992)	(6,365)	(6,637)	4,583	(1,992)		3,209
AXT, Inc. Stockholders' Equity
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	199,783	195,149	192,619	188,362	187,016	187,885	192,619	187,885
Common stock options exercised	468	420	704	624	72	441
Stock-based compensation	1,639	975	816	648	641	643
Formation of new subsidiary with noncontrolling interests		(131)	(131)	396
Sale of common stock to employees in connection with the reorganization			538
Purchase of subsidiary shares from noncontrolling interest			(2,691)
Transfer of subsidiary with noncontrolling interest		(1,229)
Transfer of subsidiary with redeemable noncontrolling interest		(1,241)
Net income (loss)	3,800	4,385	3,425	991
Net income (loss)					361	(178)
Other comprehensive income (loss)	50	1,455	(131)	4,373	272	(1,775)
Balance, end of period	205,740	199,783	195,149	195,394	188,362	187,016	205,740	195,394		192,619
Noncontrolling Interests
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	17,479	15,313	15,350	5,692	5,174	4,877	15,350	4,877
Formation of new subsidiary with noncontrolling interests		706	707
Purchase of subsidiary shares from noncontrolling interest			(1,039)
Transfer of subsidiary with noncontrolling interest		1,229
Net income (loss)	329	94	311	490
Net income (loss)					598	395
Net dividend declared by joint ventures					(89)
Other comprehensive income (loss)	6	137	(16)	316	9	(98)
Balance, end of period	$ 17,814	$ 17,479	$ 15,313	$ 6,498	$ 5,692	$ 5,174	$ 17,814	$ 6,498		$ 15,350